CN115979596A - A method of pairing low-stress lenses to form a lens group and optical measuring equipment - Google Patents

Abstract

The invention discloses a method for pairing low-stress lenses to form a lens group and an optical measuring device. The method includes measuring the stress magnitude and the fast axis direction of each candidate lens, and selecting two candidate lenses with close stresses to pair to form a lens group; During the process of installing the lens group on the ellipsometer, the angle of the fast axis direction of the incident lens is a, and the fast axis direction of the exit lens is adjusted to 90°+a or 90°‑a. The lens group obtained by this 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 minimized.

Description

A method of pairing low-stress lenses to form a lens group and optical measuring equipment

technical field

The invention relates to the field of optical measurement, in particular to a method for pairing low-stress lenses to form a lens group and an optical measurement device.

Background technique

The spectroscopic ellipsometer is an important tool for detecting the film thickness of semiconductor wafers. In order to detect a smaller area, a pair of lens groups are required in the ellipsometer to focus on the sample, and then collimate after reflection from the sample. The lens or lens group is a commonly used optical device on the ellipsometer. The stress of the lens and lens group will have a certain impact on the measurement results, especially in the ultraviolet band.

In the lens, due to the stress of the lens material itself, the glue between the lenses may generate stress, and the lens may also generate stress when the lens is installed in the lens barrel. Therefore, the newly produced lens needs to be placed for a period of time to eliminate or reduce the stress. But even if it is left for a period of time, the lens still cannot avoid residual stress. In order to achieve the best measurement results of the ellipsometer, it is necessary to select a lens with less stress. In addition, it is necessary to match the two lenses according to a certain angle, so that the total Mueller matrix of the lens group is close to the identity matrix.

Contents of the invention

In order to solve the problems existing in the prior art, the present invention provides a method and optical measurement equipment for forming a lens group by pairing low-stress lenses. The method adopts a direct-through Mueller matrix ellipsometer to measure the Mueller matrix and the fast axis direction of the lens, and According to the measurement results, the lenses are paired to obtain a lens group, which reduces the influence of the stress of the lens on the measurement effect of the ellipsometer.

The technical solution of the present invention includes a method for pairing low-stress lenses to form a lens group, which is characterized in that it includes: measuring the stress magnitude and fast axis direction of each candidate lens, and selecting two candidate lenses with similar stresses to form a lens group by pairing; When the lens group is installed on the ellipsometer, the angle of the fast axis direction of the incident lens is a, and the fast axis direction of the outgoing lens is adjusted to 90°+a or 90°-a.

The further improvement of the present invention lies in: adopting the Mueller matrix and the fast axis direction of the direct-through Mueller matrix ellipsometer to measure the lens, and using the M34 element or M43 element in the Mueller matrix when the fast axis of the lens is the same as the reference direction to characterize the stress size of the lens .

The further improvement of the present invention is that: the stresses of the two lenses are close, which means that the difference between the M34 element or the M43 element of the Mueller matrix of the two lenses is smaller than the threshold value corresponding to the low stress standard.

The further improvement of the present invention is: use one of the paired two lenses as the incident lens and the other as the exit lens, when the fast axis direction of the incident lens is the same as the reference direction and the fast axis direction of the exit lens is parallel to the slow axis direction of the incident lens , measure the overall Mueller matrix of the lens group, and judge whether the matrix is an identity matrix.

The further improvement of the present invention is: the Mueller matrix when the fast axis of the lens is directed to a certain angle is measured by a straight-through Mueller matrix ellipsometer, which specifically includes: parallel light incidents sequentially through the polarizer, glass slide C1, the lens to be measured, The single lens, slide C2, and analyzer are incident on the detector. During the measurement, slide C1 and slide C2 rotate according to a predetermined speed ratio; the detector is a spectrometer, which measures slide C1, slide The spectrometer of C2 at different angles obtains the Fourier coefficients of each order, and obtains the Mueller matrix of the lens when the fast axis points to the current angle according to the Fourier coefficients of each order.

The further improvement of the present invention is: by rotating the lens, measure the Mueller matrix when the fast axis of the lens points to different directions; same.

A further improvement of the present invention is that: the reference direction is the polarization direction of the polarizer used in the measurement process.

The further improvement of the present invention lies in that: the wavelength of the incident light of the polarizer is in the ultraviolet band.

A further improvement of the present invention is that: when the lens group is installed 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 direction of the polarizer.

The present invention also provides an optical measurement device, which includes a lens group obtained by using the above-mentioned method of pairing low-stress lenses to form a lens group.

Beneficial technical effects of the present invention: the lens group obtained by adopting the method of the present invention can basically eliminate lens stress, and the Mueller matrix of the lens group is close to the identity matrix, so that the influence on measurement is minimized.

Description of drawings

The above and other objects, features and advantages of exemplary embodiments of the present invention will become readily understood by reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of the invention are shown by way of illustration and not limitation, in which:

FIG. 1 is a schematic diagram of a stress detection structure of a single lens in an embodiment of the present invention;

Fig. 2 is a schematic diagram of the lens group stress detection structure in an embodiment of the present invention;

Fig. 3 is a single lens measurement m34 measurement curve in the embodiment of the present invention;

Fig. 4 is a single lens m24 measurement curve in an embodiment of the present invention;

Fig. 5 is the lens group m24 and m34 after pairing in the embodiment of the present invention;

FIG. 6 is a paired lens group m42 and m43 in an embodiment of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the specific embodiments described here are only used to explain the present invention, rather than limit the present invention. In addition, it should be noted that, for the convenience of description, the drawings and specific embodiments only show the relevant part of the present invention rather than the whole content.

Embodiments of the present invention specifically include a method for pairing low-stress lenses to form a lens group. The method specifically includes: measuring the stress magnitude and fast axis direction of each candidate lens, and selecting two candidate lenses with close stresses to form a lens group by pairing. ; When the lens group is installed on the ellipsometer, the angle of the fast axis direction of the incident lens is a, and the fast axis direction of the outgoing lens is adjusted to 90°+a or 90°-a.

In the embodiment of the present invention, the Mueller matrix and the fast axis direction of the lens are measured by a direct Mueller matrix ellipsometer, and the M34 element or M43 element in the Mueller matrix when the fast axis of the lens is the same as the reference direction is used to characterize 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. For a stressed lens, it can be described by the Mueller matrix of the glass slide model. When the fast axis of the lens is along the reference direction, its Mueller matrix It can be expressed as:

Among them, δ is the phase delay of the fast and slow axes. Generally speaking, the stress of the lens is small (δ is small), cosδ is very close to 1 in the whole band, and sinδ is a small number.

The stress of the two lenses is close, which means that the difference between the M34 element or the M43 element of the Mueller matrix of the two lenses is smaller than the threshold value corresponding to the low stress standard.

In order to pair to form a lens group, first measure the Mueller matrix when the fast axis of each candidate lens is in the same direction as the reference. At this time, the element m34(sinδ) of the Mueller matrix can be used to represent the stress of the lens. In order to measure the fast axis direction and stress of a single lens, a straight-through Mueller matrix ellipsometer structure as shown in Figure 1 can be used. The incident parallel light passes through the polarizer P, the glass slide C1, the lens to be tested L1, and the single lens L2. The slide C2 and the analyzer A are incident on the detector (spectrometer), and the slides C1 and C2 rotate according to a certain speed ratio. The spectra of C1 and C2 at different angles are measured by the spectrometer, and the Fourier of each order can be obtained. Leaf coefficients (a ₂ ～α ₂₄ , β ₂ ～β ₂₄ ), the total Mueller matrix composed of the lens L1 and the single lens L2 can be solved from the Fourier coefficients. Because the stress of the single lens is very small (only the stress of the lens material itself, no glue between the lenses, and the stress caused by the lens installed in the lens barrel), it can be ignored, and its total Mueller matrix is very close to the Mueller matrix of the lens L1, which can be used to replace the Mueller matrix of lens L1. By rotating the lens L1, the Mueller matrix of the lens at different angles can be obtained. When the element m24 of the Mueller matrix of the lens L1 is close to 0 in the whole band (as shown in Figure 4), the fast axis direction of the lens is the same as the reference direction (the reference direction is the polarization direction of the polarizer used in the measurement process), The m34 curve shows the stress (sinδ) of the lens L1 at different wavelengths. Generally speaking, the m34 of a single lens is small in the visible band and increases in the ultraviolet band, as shown in Figure 3.

After detecting the fast axis direction of a single lens, two lenses need to be paired to form a lens group. The choice of two lenses should select two lenses with the same stress (or very close, that is, when the fast axis of the lens is in the reference direction, the shape and size of the m34 curve are very close) for pairing, otherwise a better pairing result cannot be obtained (That is, the total Mueller matrix of the obtained lens group is close to the identity matrix, and its elements m24, m34, m42, and m43 are all close to 0).

The paired detection structure of the two lenses is shown in Figure 2: the incident parallel light passes through the polarizer P, the glass slide C1, the lens L1, the lens L2, the glass slide C2, and the analyzer A, and then enters the detector (spectrometer). The slices C1 and C2 rotate according to a certain speed ratio, and the spectra of C1 and C2 at different angles are measured by the spectrometer, and the Fourier coefficients of each order (a ₂ ～α ₂₄ , β ₂ ～β ₂₄ ) can be obtained. The leaf coefficients can be solved for the total Mueller matrix composed of shot L1 and shot L2. Rotating the lens L1 and the lens L2 can obtain the Mueller matrix of the lens group under different angle combinations. In order to quickly obtain the best angle combination of the two lenses, the fast axis direction of the lens L1 can be rotated to (the angle with the reference direction) 0°, and the fast axis direction of the L2 can be rotated to 90° accordingly (that is, L2=L1+90° ), so that the lens group basically achieves the best angle combination. As long as the two lenses have little difference in the full band of m34, the total Mueller matrix is close to the identity matrix. As shown in Figure 5 and Figure 6, m24, m34, m42 and m43 of the Mueller matrix of the two lenses under the optimal angle combination, all four elements are close to 0 in the whole band.

The lens combination selected above is paired without the sample. When the lens group is installed on the ellipsometer for measurement, the total Mueller matrix of the combination of lens L1+sample+lens L2 is required to be equal to (or very close to) the sample itself The Mueller matrix, so that the measured sample parameters (n, k and thickness) are closer to the real sample parameters. The angle of the lens group installed on the ellipsometer tooling should meet the following rules, assuming that the angle between the fast axis direction of the lens L1 and the reference direction is a, then the angle between the lens L2 and the reference direction should be 90°±a, preferably is 90°-a. In addition, in order to better eliminate the influence of lens group stress, the angle a of the fast axis direction of the lens L1 may be the same as the polarization angle P0 of the polarizer. For example, when P0=20°, the angle of the lens L1 can be 20°, and the angle of the lens L2 can be 90°-20°=70°.

Embodiments of the present invention also include an optical measurement device, which includes a lens group obtained by using the method for forming a lens group by pairing low-stress lenses.

The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person familiar with the technical field can easily think of various equivalents within the technical scope disclosed in the present invention. Modifications or replacements shall all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims (10)

1. A method for pairing low-stress lenses to form a lens group, characterized in that it comprises: measuring the stress size and the fast axis direction of each candidate lens, and selecting two candidate lenses whose stresses are close to pairing to form a lens group; When the group is installed on the ellipsometer, the angle of the fast axis direction of the incident lens is a, and the fast axis direction of the exit lens is adjusted to 90°+a or 90°-a. 2. The method for pairing low-stress lenses according to claim 1 to form a lens group is characterized in that: the Mueller matrix and the fast axis direction of the lens are measured by a straight-through Mueller matrix ellipsometer, and the fast axis of the lens is aligned with the reference direction. At the same time, the M34 element or M43 element in the Mueller matrix represents the stress of the lens. 3. The method for pairing low-stress lenses to form a lens group as claimed in claim 2, wherein the stresses of the two lenses are close, meaning that the difference between the M34 element or the M43 element of the Mueller matrix of the two lenses is less than the low-stress standard the corresponding threshold. 4. The method for pairing low-stress lenses to form a lens group as claimed in claim 1, wherein one of the two paired lenses is used as an incident lens and the other as an outgoing lens, when the fast axis direction of the incident lens and the reference direction When the same and the fast axis direction of the outgoing lens is parallel to the slow axis direction of the incoming lens, measure the Mueller matrix of the lens group as a whole and judge whether the matrix is an identity matrix. 5. The method for forming a lens group by pairing low-stress lenses as claimed in claim 2 or 4, wherein the Mueller matrix when the fast axis of the lens is directed to a certain angle is measured by a straight-through Mueller matrix ellipsometer specifically includes: parallel light The incidence is incident on the detector after passing through the polarizer, slide C1, the lens to be tested, single lens, slide C2, and analyzer in sequence. During the measurement, slide C1 and slide C2 follow a predetermined speed ratio Rotate; the detector is a spectrometer, which obtains the Fourier coefficients of each order by measuring the spectrometer of the slide C1 and the slide C2 at different angles, and obtains the lens pointing at the fast axis according to the Fourier coefficients of each order. Mueller matrix at the current angle. 6. The method for forming a lens group by pairing low-stress lenses as claimed in claim 5, wherein, by rotating the lens, measuring the Mueller matrix when the fast axis of the lens points to different directions; when the M24 element of the Mueller matrix of the lens is in each measurement When the bands are close to 0, the fast axis direction of the lens is the same as the reference direction. 7. The method for pairing low-stress lenses to form a lens group according to claim 5, wherein the reference direction is the polarization direction of the polarizer used in the measurement process. 8 . The method for pairing low-stress lenses to form a lens group according to claim 7 , wherein the wavelength of the incident light of the polarizer is in the ultraviolet band. 9 . The method for pairing low-stress lenses to form a lens group according to claim 7 , wherein when the lens group is installed on the ellipsometer, the direction of the fast axis of the incident lens is parallel to the direction of the polarizer. 10. An optical measurement device, characterized in that it comprises a lens group obtained by using the method of pairing low-stress lenses to form a lens group according to any one of claims 1 to 9.

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