Apparatus and method for detecting wavefront aberration of objective lens

Abstract

Apparatus and method for detecting wavefront aberration of an objective lens, comprising a wavefront detection system, a planar mirror, and a planar mirror adjusting mechanism; the objective lens is placed between planar mirror and wavefront detection system; planar mirror is positioned at focal point of the objective lens. A test wavefront emitted by wavefront detection system passes through the objective lens, gets reflected by the planar mirror, and t passes through the objective lens again; the wavefront detection system receives and detects the test wavefront to derive a phase distribution thereof; an angle of the planar mirror tilts at is adjusted to obtain different return wavefronts; a polynomial for expressing wavefront aberration is selected, and expressions corresponding to all the return wavefronts are calculated; result of fitting the wavefront aberration of the objective lens when expressed by the selected polynomial is derived through fitting with the polynomial.

Claims

1. A method for detecting wavefront aberration of an objective lens to be detected by using an apparatus comprising a wavefront detection system (1), a planar mirror (3) with an angle the planar mirror (3) tilts at relative to an optical axis of the wavefront detection system (1), a planar mirror adjusting mechanism (4) bearing the planar mirror (3), and an objective lens (2) placed between the wavefront detection system (1) and the planar mirror (3), comprising: step (1) enabling a test wavefront emitted by the wavefront detection system (1) to pass through the objective lens (2) and using (x.sub.IN,y.sub.IN) to denote an intersection point of the test wavefront and a pupil plane of the objective lens (2); step (2) adjusting the planar mirror adjusting mechanism (4) so that the planar mirror (3) is positioned at a focal point of the objective lens (2), wherein the test wavefront passes through the objective lens (2) and arrives upon the planar mirror (3), a reflected wavefront reflected by the planar mirror (3) passes through the objective lens (2) again and is received by the wavefront detection system (1); and using (x1.sub.OUT, y1.sub.OUT) to denote an intersection point of the reflected wavefront and the pupil plane of the objective lens (2); step (3) detecting, by the wavefront detection system (1), the reflected wavefront, calculating and deriving phase information Wt1(x1.sub.OUT, y1.sub.OUT)=W.sub.lens(x.sub.IN, y.sub.IN)+W.sub.lens(x1.sub.OUT, y1.sub.OUT), wherein W.sub.lens is wavefront aberration of the objective lens (2); step (4) adjusting, by the planar mirror adjusting mechanism (1), so that the planar mirror (3) is positioned at the focal point of the objective lens (2) and the angle the planar mirror (3) tilts at is changed, wherein the wavefront detection system (1) receives and detects the reflected wavefront Wt2 when the angle is changed; step (5) repeating step (4), recording the reflected wavefronts Wtn received and detected by the wavefront detection system (1) under different angles the planar mirror (3) tilts at, wherein n is an integer that is greater than or equal to 3 and indicates a sequence of the different angles; step (6) selecting an m-term polynomial P.sub.i for fitting the wavefront aberration of the objective lens (2), and calculating a polynomial Pn.sub.i(xn.sub.OUT, yn.sub.OUT)=P.sub.i(c.sub.IN,y.sub.IN)+P.sub.i(xn.sub.OUT, yn.sub.OUT), i=1˜m corresponding to each reflected wavefront Wtn according to the angle the planar mirror (3) tilts at; step (7) calculating a polynomial coefficient C.sub.m of the wavefront aberration of the objective lens (2) corresponding to the polynomial P.sub.m through a formula as follows: $.Math.\begin{array}{cccc}\mathrm{Pcol}{1}_1& \mathrm{Pcol}{1}_2& .Math.& \mathrm{Pcol}{1}_m\\ \mathrm{Pcol}{2}_1& \mathrm{Pcol}{2}_2& .Math.& \mathrm{Pcol}{2}_m\\ .Math.& .Math.& \ddots & .Math.\\ {\mathrm{Pcoln}}_1& {\mathrm{Pcoln}}_2& .Math.& {\mathrm{Pcoln}}_m\end{array}.Math..Math.\begin{array}{c}{C}_1\\ C_2\\ .Math.\\ C_m\end{array}.Math.=.Math.\begin{array}{c}\mathrm{Wtcol}1\\ \mathrm{Wtcol}2\\ .Math.\\ \mathrm{Wtcoln}\end{array}.Math..$ wherein Pcoln.sub.i and Wtcoln are column vectors of Pn.sub.i and Wtn, respectively; and step (8) deriving a result of fitting expressed by polynomial P.sub.i on basis of the polynomial coefficient C.sub.m.

2. The method for detecting wavefront aberration according to claim 1, wherein at least three sets of return wavefronts Wt1, Wt2, Wt3, under different angles are measured to obtain the wavefront aberration of the objective lens (2) through fitting.

3. The method for detecting wavefront aberration according to claim 1, wherein the m-term polynomial P.sub.i is a Zernike polynomial or a Legendre polynomial.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the apparatus for detecting wavefront aberration of an objective lens of the present invention.

(2) FIG. 2 shows the return wavefront collected when an angle a planar mirror tilts at is 0 in Example 1.

(3) FIG. 3 shows the return wavefront collected when the angle the planar mirror tilts at is 1.457° in direction X of a detector in Example 1.

(4) FIG. 4 shows the return wavefront collected when the angle the planar mirror tilts at is 2.343° in direction Y of the detector in Example 1.

(5) FIG. 5 shows the result of fitting the wavefront aberration of the objective lens to be detected in Example 1.

(6) FIG. 6 shows the fitting coefficient for the wavefront aberration of the objective lens to be detected in Example 1.

(7) FIG. 7 shows the return wavefront collected when the angle the planar mirror tilts at is 0 in Example 2.

(8) FIG. 8 shows the return wavefront collected when the angle the planar mirror tilts at is 1.457° in direction X of a detector in Example 2.

(9) FIG. 9 shows the return wavefront collected when the angle the planar mirror tilts at is 2.343° in direction Y of the detector in Example 2.

(10) FIG. 10 shows the result of fitting the wavefront aberration of the objective lens to be detected in Example 2.

(11) FIG. 11 shows the fitting coefficient for the wavefront aberration of the objective lens to be detected in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

(12) The present invention is further described below with reference to examples and drawings, but the scope thereof should not be limited thereto.

Example 1

(13) As shown in FIG. 1, in the apparatus for detecting wavefront aberration of an objective lens of the present invention, a test wavefront emitted by a DynaFiz interferometer 1 passes through a microscope objective 2 to be detected (NA=0.14, with a focal length of 40 mm) and is converged at a focal point. A planar mirror 3 is fixed on an adjusting mechanism 4, and the adjusting mechanism 4 has the functions of three-dimensional displacing, and pitch and yaw regulating. The adjusting mechanism 4 adjusts so that the planar mirror 3 is positioned at the focal point, and the mirror plane is parallel to the focal plane. The wavefront passing through the microscope objective 2 to be detected is reflected by the planar mirror 3 at the focal point, gets reversed by 180°, and then passes through the microscope objective 2 to be detected again. The aberration brought about by the former and later return wavefronts that pass through the microscope objective 2 to be detected is the result of a doubled wavefront aberration of the microscope objective 2 to be detected after the 180° reversion. The return wavefront is detected and recorded by the DynaFiz interferometer 1 as shown in FIG. 2.

(14) The adjusting mechanism 4 adjusts so that planar mirror 3 is still positioned at the focal point, but tilts at an angle of 1.457° in direction X of a pixel coordinate system of the DynaFiz interferometer 1 detector. At the time, after the wavefront passing through the microscope objective 2 to be detected is reflected by the planar mirror 3 at the focal point, only a partial wavefront can pass through the microscope objective 2 to be detected again and get detected by the DynaFiz interferometer 1. The detected wavefront is shown in FIG. 3.

(15) The adjusting mechanism 4 adjusts so that planar mirror 3 is still positioned at the focal point, but tilts at an angle of 2.343° in direction Y of a pixel coordinate system of the DynaFiz interferometer 1 detector. At the time, after the wavefront passing through the microscope objective 2 to be detected is reflected by the planar mirror 3 at the focal point, only a partial wavefront can pass through the microscope objective 2 to be detected again and get detected by the DynaFiz interferometer 1. The detected wavefront is shown in FIG. 4.

(16) A 100-term Zernike polynomial is taken to represent the wavefront aberration of the microscope objective lens 2 to be detected, and the polynomial expressions corresponding to the return wavefronts shown in FIGS. 2 to 4 are calculated. The derived polynomial expressions and the return wavefronts are subjected to least-square fitting to obtain a result of fitting the wavefront aberration of the microscope objective lens 2 to be detected, and a corresponding result of coefficients of the 100-term Zernike polynomial is obtained, which are shown in FIGS. 5 and 6, respectively.

Example 2

(17) As shown in FIG. 1, in the apparatus for detecting wavefront aberration of an objective lens of the present invention, a test wavefront emitted by a DynaFiz interferometer 1 passes through a microscope objective lens 2 to be detected (NA=0.9, with a focal length of 1.8 mm) and is converged at a focal point. A planar mirror 3 is fixed on an adjusting mechanism 4, and the adjusting mechanism 4 has the functions of three-dimensional displacing, and pitch and yaw regulating. The adjusting mechanism 4 adjusts so that the planar mirror 3 is positioned at the focal point, and the mirror plane is parallel to the focal plane. The wavefront passing through the microscope objective 2 to be detected is reflected by the planar mirror 3 at the focal point, gets reversed by 180°, and then passes through the microscope objective 2 to be detected again. The aberration brought about by the former and later return wavefronts that pass through the microscope objective 2 to be detected is the result of a doubled wavefront aberration of the microscope objective 2 to be detected after the 180° reversion. The return wavefront is detected and recorded by the DynaFiz interferometer 1 as shown in FIG. 7.

(18) The adjusting mechanism 4 adjusts so that planar mirror 3 is still positioned at the focal point, but tilts at an angle of 10.501° in direction X of a pixel coordinate system of the DynaFiz interferometer 1 detector. At the time, after the wavefront passing through the microscope objective lens 2 to be detected is reflected by the planar mirror 3 at the focal point, only a partial wavefront can pass through the microscope objective lens 2 to be detected again and get detected by the DynaFiz interferometer 1. The detected wavefront is shown in FIG. 8.

(19) The adjusting mechanism 4 adjusts so that planar mirror 3 is still positioned at the focal point, but tilts at an angle of 5.893° in direction Y of a pixel coordinate system of the DynaFiz interferometer 1 detector. At the time, after the wavefront passing through the microscope objective lens 2 to be detected is reflected by the planar mirror 3 at the focal point, only a partial wavefront can pass through the microscope objective lens 2 to be detected again and get detected by the DynaFiz interferometer 1. The detected wavefront is shown in FIG. 9.

(20) A 100-term Zernike polynomial is taken to represent the wavefront aberration of the microscope objective 2 to be detected, and the polynomial expressions corresponding to the return wavefronts shown in FIGS. 7 to 9 are calculated. The derived polynomial expressions and the return wavefronts are subjected to least-square fitting to obtain a result of fitting the wavefront aberration of the microscope objective 2 to be detected, and a corresponding result of coefficients of the 100-term Zernike polynomial is obtained, which are shown in FIGS. 10 and 11, respectively.

(21) The present invention features a simple structure, requires low costs, and a detectable numerical aperture (NA) of the objective lens is not restricted by parts in the system, moreover, the result of fitting the wavefront aberration of the objective lens to be detected may have a margin of error lower than 10%.