CN115979596A - Method for forming lens group by pairing low-stress lenses and optical measurement equipment - Google Patents

Method for forming lens group by pairing low-stress lenses and optical measurement equipment Download PDF

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CN115979596A
CN115979596A CN202310057327.1A CN202310057327A CN115979596A CN 115979596 A CN115979596 A CN 115979596A CN 202310057327 A CN202310057327 A CN 202310057327A CN 115979596 A CN115979596 A CN 115979596A
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lens
stress
fast axis
mueller matrix
axis direction
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庄源
杨翼
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Raintree Scientific Instruments Shanghai Corp
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Raintree Scientific Instruments Shanghai Corp
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Abstract

The invention discloses a method for pairing low-stress lenses to form a lens group and optical measurement equipment, wherein the method comprises the steps of measuring the stress magnitude and the fast axis direction of each candidate lens, and selecting two candidate lenses with close stress to pair to form the lens group; in the process of mounting the lens group on the ellipsometer, the fast axis direction angle of the incident lens is a, and the fast axis direction of the emergent lens is adjusted to 90 degrees + a or 90 degrees-a. The lens group obtained by the method can basically eliminate the lens stress, and the Mueller matrix of the lens group is close to the identity matrix, so that the influence on the measurement is minimum.

Description

Method for forming lens group by pairing low-stress lenses and optical measurement equipment
Technical Field
The invention relates to the field of optical measurement, in particular to a method for forming a lens group by pairing low-stress lenses and optical measurement equipment.
Background
The spectrum type ellipsometer is an important tool for detecting the thickness of a semiconductor wafer, and in order to detect a smaller area, a pair of lens groups is required in the ellipsometer to focus a sample, and the sample is reflected and then collimated. The lens or the lens group is a common optical device on an ellipsometer, and the stress of the lens or the lens group has a certain influence on a measurement result, especially in an ultraviolet band.
In the lens, because the lens material has stress, the bonding between the lenses may generate stress, and the mounting of the lenses in the lens barrel may also generate stress, therefore, the newly produced lens needs to be placed for a period of time to eliminate or reduce the stress. However, even if the lens is left for a while, the residual stress of the lens cannot be avoided. In order to optimize the ellipsometer measurement, a lens with less stress needs to be selected. In addition, the two lenses need to be matched according to a certain angle, so that the total Mueller matrix of the lens group is close to the identity matrix.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a method for forming a lens group by matching low-stress lenses and optical measurement equipment.
The technical scheme of the invention comprises a method for forming a lens group by pairing low-stress lenses, which is characterized by comprising the following steps: measuring the stress magnitude and the fast axis direction of each candidate lens, and selecting two candidate lenses with close stress to pair to form a lens group; when the lens group is installed on the ellipsometer, the angle of the incident lens in the fast axis direction is a, and the fast axis direction of the emergent lens is adjusted to 90 degrees + a or 90 degrees-a.
The invention is further improved in that: and measuring the Mueller matrix and the fast axis direction of the lens by using a through Mueller matrix ellipsometer, and representing the stress of the lens by using M34 elements or M43 elements in the Mueller matrix when the fast axis of the lens is the same as the reference direction.
The invention is further improved in that: the two lenses are close in stress, which means that the difference between the M34 elements or M43 elements of the Mueller matrix of the two lenses is smaller than the threshold corresponding to the low stress criterion.
The invention is further improved in that: and when the fast axis direction of the incident lens is the same as the reference direction and the fast axis direction of the emergent lens is parallel to the slow axis direction of the incident lens, measuring the Mueller matrix of the whole lens group and judging whether the matrix is an identity matrix.
The invention is further improved in that: the Mueller matrix when the fast axis of the through Mueller matrix ellipsometer measuring lens points to a certain angle specifically comprises: parallel light is incident into a detector after sequentially passing through a polarizer, a glass slide C1, the lens to be detected, a single lens, a glass slide C2 and an analyzer, and the glass slide C1 and the glass slide C2 rotate according to a preset rotation speed ratio in the measurement process; the detector is a spectrometer, the spectrometer for measuring the glass slides C1 and C2 at different angles is used for solving Fourier coefficients of all orders, and a Mueller matrix of the lens when the fast axis points to the current angle is obtained according to the Fourier coefficients of all orders.
The invention is further improved in that: measuring a Mueller matrix when a fast axis of a lens points to different directions by rotating the lens; when the M24 element of the Mueller matrix of the lens is close to 0 at each measurement band, the fast axis direction of the lens at this time is the same as the reference direction.
The invention is further improved in that: the reference direction is the polarization direction of a polarizer used in the measurement process.
The invention is further improved in that: the wavelength of the incident light of the polarizer is in an ultraviolet band.
The invention is further improved in that: when the lens group is arranged on the ellipsometer, the direction parallel to the incident surface is taken as the reference direction, and the fast axis direction of the incident lens is parallel to the polarizer direction.
The invention also provides an optical measuring device which comprises the lens group obtained by adopting the method for forming the lens group by the low-stress lens pairing.
The invention has the beneficial technical effects that: the lens group obtained by the method of the invention can basically eliminate the lens stress, and the Mueller matrix of the lens group is close to the unit matrix, thereby minimizing the influence on the measurement.
Drawings
The above and other objects, features and advantages of exemplary embodiments of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings. Several embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which:
FIG. 1 is a schematic diagram of a single lens stress detection structure according to an embodiment of the invention;
FIG. 2 is a diagram illustrating a lens set stress detection structure according to an embodiment of the present invention;
FIG. 3 is a single lens measurement m34 measurement curve in an embodiment of the present invention;
FIG. 4 is a single lens m24 measurement curve in an embodiment of the present invention;
fig. 5 is a view of the paired rear lens groups m24 and m34 in the embodiment of the present invention;
fig. 6 is a diagram showing the paired rear lens groups m42 and m43 in the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in detail with reference to the accompanying drawings and specific embodiments. It should be noted that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It is also to be understood that the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.
Embodiments of the present invention specifically include a method for forming a lens group by pairing low-stress lenses, the method specifically including: measuring the stress magnitude and the fast axis direction of each candidate lens, and selecting two candidate lenses with close stress to pair to form a lens group; when the lens group is installed on the ellipsometer, the angle of the incident lens in the fast axis direction is a, and the fast axis direction of the emergent lens is adjusted to 90 degrees + a or 90 degrees-a.
In the embodiment of the invention, a direct-through Mueller matrix ellipsometer is adopted to measure the Mueller matrix and the fast axis direction of the lens, and M34 element or M43 element in the Mueller matrix with the fast axis of the lens being the same as the reference direction is adopted to represent the stress of the lens. The lens can be described by a Mueller matrix, the Mueller matrix of an ideal lens should be an identity matrix, and for stressed lenses can be described by a Mueller matrix of a slide model, and when the fast axis of the lens is along a reference direction, the Mueller matrix can be expressed as:
Figure BDA0004060722620000031
where δ is the fast and slow axis phase retardation, the lens stress is generally small (δ is small), cos δ is very close to 1 at all bands, sin δ is a small number.
The two lenses are close in stress, which means that the difference between M34 elements or M43 elements of the Mueller matrix of the two lenses is smaller than the threshold corresponding to the low stress criterion.
To pair to form a lens group, a Mueller matrix is first measured in which the fast axis of each candidate lens is the same as the reference direction. The element m34 (sin δ) of the Mueller matrix at this time can be used to represent the stress magnitude of the lens. In order to measure the fast axis direction and stress of a single lens, a through Mueller matrix ellipsometer structure shown in FIG. 1 can be adopted, parallel light is incident to a detector (spectrometer) after sequentially passing through a polarizer P, a glass slide C1, a lens L1 to be measured, a single lens L2, a glass slide C2 and an analyzer A, the glass slides C1 and C2 rotate according to a certain rotation speed ratio, and the spectrums of the C1 and C2 at different angles measured by the spectrometer can be used for calculating Fourier coefficients (a) of various orders 2 ～α 24 ,β 2 ～β 24 ) The total Mueller matrix consisting of the lens L1 and the single lens L2 can be solved by fourier coefficients. Because the stress of the single lens is very small (only the stress of the lens material itself, no gluing between lenses, and the stress caused by mounting the lenses on the lens barrel), it can be ignored, and the total Mueller matrix is very close to the Mueller matrix of the lens L1, and can be used to replace the Mueller matrix of the lens L1. And rotating the lens L1 to obtain Mueller matrixes of the lens under different angles. When the element m24 of the Mueller matrix of the lens L1 is close to 0 in the full band (as shown in FIG. 4), the fast axis direction of the lens is aligned with the reference direction (the reference direction is the polarization direction of the polarizer used in the measurement process)) Similarly, the m34 curve or band represents the stress magnitude (sin δ) of lens L1 at different wavelengths. In general, m34 of a single lens is smaller in the visible band and the ultraviolet band is increased, as shown in fig. 3.
After the fast axis direction of a single lens is detected, two lenses need to be paired to form a lens group. The two lenses should be selected to be paired with the same stress (or close to each other, that is, when the fast axis of the lens is in the reference direction, the shape of the m34 curve is close to each other), otherwise, a better pairing result cannot be obtained (the total Mueller matrix of the obtained lens group is close to the identity matrix, and the elements m24, m34, m42 and m43 are close to 0).
The two-lens pairing detection structure is shown in fig. 2: parallel light is incident into a detector (spectrometer) after sequentially passing through a polarizer P, a glass slide C1, a lens L2, a glass slide C2 and an analyzer A, the glass slides C1 and C2 rotate according to a certain rotation speed ratio, and the spectrometer measures spectra of the C1 and the C2 at different angles to obtain Fourier coefficients (a) of various orders 2 ～α 24 ,β 2 ～β 24 ) The total Mueller matrix consisting of shot L1 and shot L2 can be solved from the fourier coefficients. Rotating lens L1 and lens L2 can obtain a Mueller matrix with different combinations of angles of lens groups. In order to quickly obtain the optimal angle combination of the two lenses, the fast axis direction of the lens L1 can be rotated to (the included angle with the reference direction is) 0 °, and the fast axis direction of the lens L2 is correspondingly rotated to 90 ° (i.e. L2= L1+90 °), so that the lens group basically reaches the optimal angle combination, and as long as the m34 full-wave bands of the two lenses are not greatly different, the total Mueller matrix approaches the unit matrix. As shown in fig. 5 and 6 for m24, m34, m42 and m43 of the Mueller matrix with the two lenses at the optimal angular combination, all 4 elements are close to 0 at full band.
The lens combination selected above is matched without a sample, when the lens set is installed on an ellipsometer for measurement, the total Mueller matrix of the lens L1+ sample + lens L2 combination is required to be equal to (or very close to) the Mueller matrix of the sample itself, so that the measured sample parameters (n, k and thickness) are closer to the real sample parameters. The angle at which the lens group is mounted on the ellipsometer tool should satisfy the following rule, assuming that the included angle between the fast axis direction of the lens L1 and the reference direction is a, the included angle between the lens L2 and the reference direction should be 90 ° ± a, and preferably 90 ° -a. In addition, in order to better eliminate the effect of the stress of the lens group, the fast axis direction angle a of the lens L1 may be the same as the polarization angle P0 of the polarizer. For example, when P0=20 °, the lens L1 angle may be 20 °, and the lens L2 angle may be 90 ° -20 ° =70 °.
Embodiments of the present invention also include an optical measurement device including a lens group obtained using the above-described method of forming a lens group using a low stress lens pairing.
While the invention has been described with reference to specific embodiments, the invention is not limited thereto, and various equivalent modifications and substitutions can be easily made by those skilled in the art within the technical scope of the invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (10)

1. A method for forming a lens assembly of a low stress lens pair, comprising: measuring the stress magnitude and the fast axis direction of each candidate lens, and selecting two candidate lenses with close stress to pair to form a lens group; in the process of mounting the lens group on the ellipsometer, the fast axis direction angle of the incident lens is a, and the fast axis direction of the emergent lens is adjusted to be 90 degrees + a or 90 degrees-a.
2. The method for forming a lens assembly in a low stress lens pair of claim 1, further comprising: and measuring the Mueller matrix and the fast axis direction of the lens by using a through Mueller matrix ellipsometer, and representing the stress magnitude of the lens by using an M34 element or an M43 element in the Mueller matrix when the fast axis of the lens is the same as the reference direction.
3. The method as claimed in claim 2, wherein the two lenses are close in stress, which means that the difference between M34 elements or M43 elements of the Mueller matrix of the two lenses is smaller than the threshold corresponding to the low stress standard.
4. The method of forming a lens assembly in a low stress lens pair of claim 1, further comprising: and when the fast axis direction of the incident lens is the same as the reference direction and the fast axis direction of the emergent lens is parallel to the slow axis direction of the incident lens, measuring the Mueller matrix of the whole lens group and judging whether the matrix is an identity matrix.
5. The method of claim 2 or 4, wherein the step of measuring the Mueller matrix when the fast axis of the lens is pointing at a certain angle by using a straight-through Mueller matrix ellipsometer comprises: parallel light is incident into a detector after sequentially passing through a polarizer, a glass slide C1, the lens to be detected, a single lens, a glass slide C2 and an analyzer, and the glass slide C1 and the glass slide C2 rotate according to a preset rotation speed ratio in the measurement process; the detector is a spectrometer, the Fourier coefficients of all orders are obtained by measuring the spectrometers of the glass slide C1 and the glass slide C2 at different angles, and a Mueller matrix of the lens when the fast axis points to the current angle is obtained according to the Fourier coefficients of all orders.
6. The method of claim 5, wherein the Mueller matrix is measured by rotating the lens with the fast axis of the lens pointing in different directions; when the M24 element of the Mueller matrix of the lens is close to 0 at each measurement band, the fast axis direction of the lens is the same as the reference direction at this time.
7. The method as claimed in claim 5, wherein the reference direction is a polarization direction of a polarizer used in the measuring process.
8. The method of forming a lens assembly in a low stress lens pair of claim 7, further comprising: the wavelength of the incident light of the polarizer is in an ultraviolet band.
9. The method of forming a lens assembly in a low stress lens pair of claim 7, further comprising: when the lens group is arranged on the ellipsometer, the fast axis direction of the incident lens is parallel to the polarizer direction.
10. An optical measuring device comprising a lens group obtained by the method of forming a lens group by pairing a low-stress lens as claimed in any one of claims 1 to 9.
CN202310057327.1A 2023-01-20 2023-01-20 Method for forming lens group by pairing low-stress lenses and optical measurement equipment Pending CN115979596A (en)

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