CN106772923B

CN106772923B - Automatic focusing method and system based on inclined slit

Info

Publication number: CN106772923B
Application number: CN201510822396.2A
Authority: CN
Inventors: 钟向红; 唐安伦; 徐益平
Original assignee: Raintree Scientific Instruments Shanghai Corp
Current assignee: Raintree Scientific Instruments Shanghai Corp
Priority date: 2015-11-24
Filing date: 2015-11-24
Publication date: 2021-01-01
Anticipated expiration: 2035-11-24
Also published as: CN106772923A

Abstract

The invention discloses a device and a method for focusing, wherein the device comprises: an illumination unit comprising a light source (101), an illumination lens (102) and an illumination slit (103) for providing incident light; the beam splitting unit is used for splitting the received light; an objective lens (107) for imaging the illumination slit (103) on a surface of an object to be measured (109) and reflecting the imaging of the illumination slit (103) to the beam splitting unit; a detection unit comprising a detection slit (113) and a detector (114) for receiving the imaging of the illumination slit (103) from the beam splitting unit and recording a confocal signal related to the surface of the object to be measured (109), wherein the detection slit (113) and the illumination slit (103) are respectively arranged obliquely with the same inclination angle with respect to the respective optical axes and in a confocal position with respect to the objective lens (107).

Description

Automatic focusing method and system based on inclined slit

Technical Field

The invention belongs to the field of semiconductor measurement, and particularly relates to an automatic focusing method and system of wafer detection equipment with patterns on the surface.

Background

In the manufacturing process of an integrated circuit chip, the product yield is an important index for measuring the chip manufacturing process. As the critical dimension of integrated circuits is continuously reduced and the pattern density is continuously increased, the difficulty of process control is also increased, and defects that could be ignored before now may cause the devices to fail to work normally, which becomes fatal defects affecting the yield. Meanwhile, new three-dimensional device structures, such as finfets, and new semiconductor materials (e.g., copper interconnects, low-k and high-k dielectric materials, and metal gate electrodes) are widely used, leading to new electrical performance defect issues; new process flows, such as immersion lithography, introduce new defect types, including particles, bubbles, watermarks, or bridges; CMP planarization brings about new defects, etc., and the existence of the defects may cause the device to fail to operate properly, which becomes an important factor affecting yield. In order to improve the process and to increase the production yield, the detection and the analytical evaluation of defects are a necessary and important task in the process control.

The most common detection equipment is optical microscopic imaging equipment, which analyzes and identifies the difference between the device to be detected and the original design index, such as the critical dimension, defect-free and the distribution position thereof, after a series of process manufacturing processes by recording the image data of the surface of the wafer with the pattern. The patterned wafer surface pattern structure is typically periodic and includes a plurality of repeating rectangular cells, i.e., work cells, surrounded by horizontal and vertical reticle boundaries that internally define the desired semiconductor integrated circuit pattern. The wafer surface diameter is typically large (200 or 300mm) and the field of view of the optical microscope is typically less than 1 mm. Therefore, to perform microscopic imaging on the entire wafer surface, a segmented scanning and regrouping method is required. Since the critical dimension of advanced integrated circuits has entered the range of tens of nanometers, high resolution (high power high Numerical Aperture (NA)) objective lenses must be used, but the depth of focus is very short, typically only 200 nm and 300 nm. During the detection scanning process, clear image data can be obtained by ensuring that the surface of the observed area is positioned in the focal plane of the objective lens or the focal depth range of the objective lens, and image blurring can be caused by defocusing. The defocusing is caused by various factors, such as unevenness of the wafer surface or inclination of the whole wafer, and especially when the wafer is fixed on a vacuum chuck, the vacuum suction force can cause deformation of the wafer surface, and the surface depression depth can reach tens of microns and is far larger than the focal depth length. Therefore, an auto-focusing technique is indispensable in the scanning process. It is a key technology in the field of wafer inspection.

To achieve automatic focusing, it is necessary to first detect a defocus condition and then control the moving mechanism to perform a defocus compensation operation. Therefore, the core of the auto-focus technique is the defocus detection method. Defocus is directional, either positive defocus or negative defocus, i.e. whether the object is close to the objective or far away from the objective. Therefore, the defocus detection method adopted by the auto-focus technology must be capable of simultaneously detecting the magnitude and direction of defocus, and the auto-focus technology used in the wafer detection equipment must also be capable of overcoming the influence of the integrated circuit pattern on the surface of the wafer and the reflectivity difference of the material.

In order to realize automatic focusing during wafer inspection, the following automatic focusing techniques can be generally adopted:

(1) patent US4639587 discloses that a projection grating is illuminated alternately by two light sources arranged off-axis from left to right, another same grating is used as a detection grating, a detector is used to receive the total light intensity signals of the left and right light beams passing through the detection grating alternately, the defocus information is obtained by analyzing the difference between the two signals, and then defocus compensation is performed accordingly to achieve focusing. Because the two paths of light are not simultaneously detected, in order to eliminate different influences of instantaneous fluctuation of the light source on detection results of the two paths of light, a light source beam splitting branch is added to detect the fluctuation of the light source in real time. This method is complicated to implement both structurally and in control.

(2) Patent US7961763B2 discloses that a linear light source inclined to the optical axis is used for projection, the reflected image is divided into two paths, the two paths are respectively received by linear detectors with two detection surfaces perpendicular to the optical axis, the two detectors are respectively located at front and back symmetrical positions of an image plane to form a differential structure, the defocusing information is obtained by analyzing the difference between the two paths of signals, and focusing is achieved accordingly. When the method is implemented, two sets of receiving light paths and detection systems are needed, and the hardware cost is high.

(3) Patent US7142315 discloses a focusing method using a tilted slit confocal optical path for the detection of the surface of the multi-layer structure, which uses two tilted slits as an illumination slit and a detection slit, respectively, which are located in a confocal position with respect to the focal plane of the objective lens. The method has the basic key points that: firstly, film layers with different depths on the surface of the wafer correspond to different pixel points on the oblique slit image; and secondly, acquiring a relative depth map between the film layers by analyzing the positions of peak pixel points in the slit confocal image. The method is premised on that: the entire wafer surface film layer must be perpendicular to the optical axis of the objective lens. In fact, the method is difficult to be practically satisfied due to the wafer surface deformation caused by the wafer inclination, especially vacuum suction. In addition, the position detection of slit confocal image peak pixel points is affected by the nonuniformity of slit illumination and the difference of the reflectivity of each layer of pattern, especially by the inevitable speckle noise of laser illumination, so that the accuracy of the result is difficult to guarantee.

Therefore, a focusing method and system that are not affected by the light intensity fluctuation of the light source, the pattern morphology of the electronic devices on the wafer and the difference of the material characteristics, and the optical structure is simpler is needed.

Disclosure of Invention

The invention overcomes the defects of the prior art and provides a novel defocusing detection and automatic focusing method based on an inclined slit confocal optical structure and a corresponding system. The system has the advantages of simple structure, convenient control and lower cost.

In one aspect, the present invention discloses an apparatus for focusing, comprising: an illumination unit comprising an incoherent light source (101) for providing incident light, an illumination lens (102) and an illumination slit (103); the beam splitting unit is used for splitting the received light; an objective lens (107) for imaging the illumination slit (103) on a surface of an object to be measured (109) and reflecting the imaging of the illumination slit (103) to the beam splitting unit; a detection unit comprising a detection slit (113) and a line detector (114) for receiving an image of the illumination slit (103) from the beam splitting unit and recording a confocal signal related to the surface of the object to be measured (109), wherein the detection slit (113) and the illumination slit (103) are respectively arranged obliquely with the same inclination angle with respect to the vertical plane of the respective optical axis, and the centers of the detection slit (113) and the illumination slit (103) are in an optically conjugate position with respect to the focal point of the objective lens (107).

In another aspect, the present invention provides an auto-focusing method, which includes: a. performing Z-axis step scanning on a measuring point of an object to be measured, and acquiring confocal signals related to the measuring point and an illumination slit through a detection slit, wherein the illumination slit and the detection slit are respectively obliquely arranged at the same inclination angle relative to the vertical plane of each optical axis; b. determining a defocus function associated with the measurement point based on the confocal signal associated with the measurement point, and linearly fitting a curve of the defocus function; c. measuring the object to be measured based on the defocusing function after linear fitting

The invention provides an automatic focusing method and system based on a tilted slit confocal method for detecting a patterned wafer. The method adopts an inclined slit confocal light path structure, uses a specific signal processing algorithm to analyze slit confocal signals to detect the magnitude and direction of defocusing amount, and then controls a related motion mechanism to execute defocusing compensation in real time, thereby realizing automatic focusing. The automatic focusing method has the advantages that the automatic focusing method is not influenced by light intensity fluctuation of a light source, the shape and the shape of a wafer pattern and material characteristic difference, compared with the prior art, the device and the structure used by the embodiment of the system are relatively simple, and the cost is lower because only one set of light path receiving and detecting system is needed.

Drawings

Other features, objects and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments thereof, when read in conjunction with the accompanying drawings.

FIG. 1 is a diagram showing an arrangement of an autofocus system based on a tilted slit confocal method according to an embodiment of the present invention;

FIG. 2a is a schematic diagram of defocus detection principles according to an embodiment of the present invention;

FIG. 2b is a plot of defocus functions in accordance with an embodiment of the present invention; and

FIG. 3 is a block diagram of an auto-focus process according to an embodiment of the present invention.

In the drawings, like or similar reference numbers indicate like or similar devices (modules) or steps throughout the different views.

Detailed Description

The invention provides a novel out-of-focus detection and automatic focusing method and a corresponding system. The automatic focusing method provided by the invention is not only not influenced by the light intensity fluctuation of the light source, but also not influenced by the shape of the wafer pattern and the reflectivity difference, and is simpler in optical structure than the method in the patents of US4639587 and US7961763B 2.

Fig. 1 shows an embodiment of the tilted slit confocal based auto-focusing system of the present invention. The hardware configuration of this embodiment is as follows: the illumination section is composed of a light source 101 and an illumination lens 102, and the light source 101 is an incoherent light source (such as an LED). The tilted slit confocal imaging optical system is composed of an illumination slit 103, a lens 104, a beam splitter 105, an objective lens 107, a lens 112, and a detection slit 113. The arrangement of the lens 104 with respect to the illumination slit 103 is identical to the arrangement of the lens 112 with respect to the detection slit 113. The illumination slit 103 and the detection slit 113 are identical and each is placed obliquely (not perpendicularly) with respect to the vertical plane of the respective optical axis at the same inclination angle, the centers of the detection slit 113 and the illumination slit 103 being in an optically conjugate position with respect to the focal point of the objective lens 107. The two slits are centered at the focal points of lens 104 and lens 112, respectively, and thus in confocal relation with the focal point of objective lens 107. The illumination slit 103 and the detection slit 113 are oriented such that their projection onto the surface to be measured via the objective lens 107 is parallel to the detection scan direction. The detector 114 is a line detector (CCD or CMOS) whose detecting surface is also inclined to the optical axis for receiving the confocal optical signal transmitted through the detecting slit 113, and the output electrical signal is transmitted to the computer 115, which is responsible for signal processing and control. In an autofocus system, dichroic beamsplitter 106 is typically a necessary component because the autofocus optical path and the detection optical path share the same objective lens 107. The dichroic beam splitter 106 functions to transmit the autofocus operating wavelength and reflect the measurement operating wavelength, which may comprise a set of different wavelengths, so that the two wavelengths are separated and do not interfere with each other. The object of the auto-focus and inspection is a patterned wafer 109, which is typically mounted on a vacuum chuck 110, which is mounted on a three-axis translation stage 111 whose X-Y-Z three-axis motion is controlled by a computer 115.

The working principle of this embodiment is: the illumination lens 102 focuses light emitted from the light source 101 on the illumination slit 103, and after the light passing through the slit passes through the lens 104, the beam splitter 105, the dichroic beam splitter 106, and the objective lens 107 in this order, a primary image 108 of the illumination slit is formed near the focal point of the objective lens 107, and the projection of the image on the surface of the wafer 109 is parallel to the scanning direction of the wafer. The image is then reflected by the surface of the wafer 109 back to the objective 107, and then sequentially passes through the dichroic beam splitter 106, the beam splitter 105 and the lens 112, so that a secondary image of the illumination slit is formed at the detection slit 113, and part of the light energy transmitted through the detection slit 113 forms a confocal signal, which is recorded by the line detector 114 and transmitted to the computer 115 for signal processing, and the magnitude and direction of the defocus amount are calculated. Subsequently, the computer 115 controls the Z-axis movement of the translation stage 111 according to the defocus amount data, and performs a defocus compensation operation to achieve focusing. And the defocusing detection and the defocusing compensation are repeatedly carried out, so that the automatic focusing in the wafer detection process can be realized. Since the detection slit 113 has a confocal relationship with the center of the illumination slit 103 and the focal point of the objective lens 107, the secondary image corresponds entirely to the primary image, and no light intensity detection is lost.

Fig. 2a is a schematic diagram illustrating a defocus detection principle according to an embodiment of the present invention.

The confocal signal detected by the line detector 114 is divided into two parts, I1 for the total intensity of the optical signal transmitted through the AB segment of the detection slit 113 and I2 for the total intensity of the optical signal transmitted through the BC segment, where AB is BC, and defines a defocus evaluation function (referred to as defocus function for short) as follows:

as shown in fig. 2a, when the surface of the wafer 109 is at the position Z0 (the focal plane of the objective lens 107), the central image point of the primary image 108 of the illumination slit intersects the surface of the wafer 109 on the optical axis Z, and the image points at the left and right sides are located outside the focal plane of the objective lens 107, and are in the positive and negative defocus states, respectively, and the defocus amounts of the left and right symmetric points are the same, but opposite. The central image point is reflected by the wafer and forms a confocal image point at the center B of the detection slit 113, the confocal signal passing through the detection slit 113 is strongest, the confocal signals passing through the detection slit 113 are weaker after the other image points are reflected by the wafer, and the confocal signal corresponding to the image point with larger defocus is weaker.

Since the total defocus amounts of the left and right portions of the primary image 108 of the illumination slit are equal, and the lengths of the wafer surface areas covered by the projections are equal, the total intensity of the confocal signals I1 transmitted through the AB segment of the detection slit 113 after being reflected is substantially equal to the total intensity of the confocal signals I2 transmitted through the BC segment, and therefore, the value of the defocus function is 0, that is, F0 is 0.

When the surface of the wafer 109 is at the positive defocus position of Z1, the intersection of the primary illumination slit image 108 and the surface of the wafer 109 moves to the right, i.e., the total defocus amount of the right half image of the primary illumination slit image 108 becomes smaller, and the total defocus amount of the left half image becomes larger. Correspondingly, the total intensity of confocal signals I1 transmitted through the AB segment of the detection slit 113 by the secondary image of the illumination slit becomes larger, and the total intensity of confocal signals I2 transmitted through the BC segment becomes smaller, when F1 > 0.

When the surface of the wafer 109 is at the negative defocus position of Z2, the intersection of the illumination slit primary image 108 and the surface of the wafer 109 moves to the left, i.e., the total defocus amount of the left half image of the illumination slit primary image 108 becomes smaller and the total defocus amount of the right half image becomes larger. Correspondingly, the total intensity of the confocal signal I1 transmitted through the AB segment of the detection slit 113 by the secondary image of the illumination slit becomes smaller, and the total intensity of the confocal signal I2 transmitted through the BC segment becomes larger, when F2 < 0.

Therefore, the direction and the amount of defocus of the objective lens 107 can be determined by the light signal intensity of the AB segment and the BC segment, and can be used for subsequent measurement.

Since I1 and I2 are the total intensity of the left and right segments of the confocal signal, and the defocus function F is only related to the ratio I1/I2 of the two segments, the defocus function F will not be affected by the fluctuation of the light source light intensity and the difference of the wafer pattern morphology and material characteristics.

It can be understood that, in this embodiment, the defocus evaluation is performed by using the difference between the optical signal intensities of the AB and BC segments, and therefore, the length of the AB segment may not be equal to the length of the BC segment.

Based on the above algorithm, the translation stage 111 is controlled to execute Z-axis step scanning, confocal signals corresponding to different defocus positions are recorded, and a defocus function curve can be obtained after data processing. Fig. 2b shows an experimental defocus function curve obtained by an embodiment of the present invention.

The positions of the defocus function values F0, F1, and F2 on the curve corresponding to the three defocus states are shown in fig. 2 b. And linear fitting is carried out on a linear area of the defocusing function curve, so that the linear relation between the defocusing amount and the defocusing function can be obtained. When the wafer is detected, in the scanning process, the confocal signal of the inclined slit is recorded in real time, the F value is calculated, and the defocusing amount Z can be calculated by utilizing the linear relation, so that defocusing detection is realized. If the computer 115 controls the Z-axis motion mechanism according to the defocus value, the defocus compensation operation is performed to achieve automatic focusing during the wafer inspection process.

For the defocus function curve in fig. 2b, the extent and slope of its linear region are related to the magnitude of the slit tilt angle. The larger the tilt angle, the smaller its linear range and the larger the slope, that is, as the tilt angle becomes larger, the focus range becomes smaller and the focus resolution becomes larger. Therefore, the actual size of the slit tilt angle should be selected according to the requirements of the system focus range and focus resolution. In the present embodiment, the slit inclination angle is 15 ° to 25 °.

FIG. 3 is a block diagram of an auto-focusing process according to an embodiment of the present invention.

The implementation process of the auto-focusing method based on the tilted slit confocal method in this embodiment includes two stages: the first stage is a calibration process, namely before the detection work starts, a focusing work curve, namely a defocusing function curve is calibrated to obtain a relational expression between the defocusing amount and the defocusing function; the second stage is a focusing process, namely after the detection work is started, a defocusing function value is measured in real time, the defocusing amount is calculated by using a calibration formula and is fed back to a Z-axis motion mechanism to execute defocusing compensation operation, and therefore automatic focusing is achieved.

The first stage is as follows: calibrating a focus work curve

Step 201: z-axis scanning and recording confocal signals

In the step, the three-dimensional translation stage is controlled by the computer to carry out Z-axis stepping scanning, so that the surface of the wafer is gradually transited from a negative defocusing state to a positive defocusing state, and the translation stage does not carry out scanning in the X-axis direction and the Y-axis direction. Synchronously, the detector records the corresponding confocal signals of the wafer surface in each out-of-focus state. The confocal signal is associated with the measurement point and the illumination slit, wherein the illumination slit and the detection slit are each arranged obliquely with the same inclination angle with respect to a vertical plane of the respective optical axis.

Step 202: calculating the defocus function

In this step, the confocal signals are processed to calculate the defocus function F value corresponding to each defocus amount using equation (1), i.e. to obtain the curve shown in fig. 2 b.

Step 203: linear fitting of defocus function

In the step, linear fitting is carried out on a linear area of the defocusing amount-defocusing function F value curve, and a linear fitting formula is obtained. For example, a fitting equation for F can be obtained that is linear with Z, i.e., F ═ k × Z + b, where k is the slope of the linear segment of the curve in fig. 2b and b is the intercept of the curve. It is to be understood that the fitting formula of F is only an example and is not intended to limit the form of the fitting.

And a second stage: focusing process

Step 204: positioning the wafer and performing X, Y axis scanning

In this step, the wafer is positioned and the three-dimensional translation stage is controlled to perform X, Y axis scanning to perform inspection imaging recording.

Step 205: recording confocal signals and calculating defocus function

In this step, the confocal signal is recorded in real time during the X, Y axis scan, and the F value is calculated based on equation (1).

Step 206: calculating defocus amount by using linear fitting formula

In this step, the defocus amount Z is determined based on the linear relationship obtained by fitting in step 203;

step 207: performing defocus compensation according to defocus amount

In the step, the Z-axis motion mechanism is controlled to execute defocusing compensation according to the defocusing amount Z, so that real-time focusing is realized.

An auto-focusing for one measurement point based on the focusing operation curve can be realized through step 204-207.

Thereafter, the auto-focus system repeats step 204-207 to scan other measurement points for real-time auto-focus throughout the disc scanning process.

It will be appreciated that the first stage focus profile calibration is a one-time task, i.e., performed prior to inspection of the first wafer, and is completed once the desired linear relationship is obtained. When all wafers are subsequently inspected, only the second stage of the defocus detection and auto-focus process from step 204 needs to be performed. In addition, the auto-focusing process in fig. 3 is not only applicable to the auto-focusing system based on the tilted slit confocal method in the foregoing embodiment, but also applicable to other auto-focusing systems without departing from the concept of the present invention.

The invention can not be influenced by light intensity fluctuation of the light source and wafer patterns when focusing, is also suitable for patterns with large reflectivity difference, and the focusing plane is the average layer of the field area of the objective lens.

According to the technical scheme, the inclined slit confocal light path structure is adopted, and automatic focusing can be realized only by one set of light path receiving and detecting system, so that the light path structure of the automatic focusing device is simplified, and the cost is reduced. In addition, the invention also adopts the analysis of the difference value of the slit confocal signals to determine the size and the direction of the defocusing amount, and then controls the relevant motion mechanism to execute defocusing compensation in real time according to the size and the direction. Therefore, the difference analysis method can eliminate the influence of light intensity fluctuation of the light source, the shape of the wafer pattern and the reflectivity difference.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. Furthermore, it will be obvious that the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. Several elements recited in the apparatus claims may also be implemented by one element. The terms first, second, etc. are used to denote names, but not any particular order.

Claims

1. An apparatus for focusing, comprising:

an illumination unit comprising an incoherent light source (101) for providing incident light, an illumination lens (102) and an illumination slit (103);

the beam splitting unit is used for splitting the received light;

an objective lens (107) for imaging the illumination slit (103) on a surface of an object to be measured (109) and reflecting the imaging of the illumination slit (103) to the beam splitting unit;

a detection unit comprising a detection slit (113) and a line detector (114) for receiving an image of the illumination slit (103) from the beam splitting unit and recording a confocal signal related to the surface of the object to be measured (109), wherein the detection slit (113) and the illumination slit (103) are respectively arranged obliquely with the same inclination angle with respect to the vertical plane of the respective optical axes, and the centers of the detection slit (113) and the illumination slit (103) are in an optically conjugate position with respect to the focal point of the objective lens (107);

wherein the focusing device is further configured to determine the defocusing direction and the defocusing amount of the objective lens according to the light signal intensity of the left segment and the right segment of the confocal signal.

2. The apparatus of claim 1, wherein the beam splitting unit comprises:

a first beam splitter (105);

a second beam splitter (106), which is a dichroic beam splitter, is arranged between the first beam splitter and the objective lens (107) for transmitting the in-focus operating wavelength and reflecting a measurement operating wavelength, which may comprise a set of different wavelengths, thereby separating the two wavelengths.

3. The apparatus of claim 1, further comprising:

a signal processing unit (115) for processing the confocal signals recorded by the line detector (114) to determine the magnitude and direction of defocus.

4. The device according to claim 1, characterized in that the illumination slit (103) and the detection slit (113) are parallel to a detection scan direction via a projection of an objective lens (107) onto the surface to be measured, and the illumination slit (103) and the detection slit (113) form an optical confocal structure with respect to the objective lens (107).

5. The apparatus of claim 3, wherein the signal processing unit (115) performs the following operations:

i. determining a defocus function of the measuring point based on the light signal intensity of the left and right two sections of confocal signals related to the surface of the object (109) to be measured, and performing linear fitting on the defocus function curve;

measuring the object (109) to be measured based on the linearly fitted defocus function.

6. The device of claim 1, wherein the tilt angle is in the range of 15 ° to 25 °.

7. An auto-focusing method, comprising:

a. performing Z-axis step scanning on a measuring point of an object to be measured, and acquiring confocal signals related to the measuring point and an illumination slit through a detection slit, wherein the illumination slit and the detection slit are respectively obliquely arranged at the same inclination angle relative to the vertical plane of each optical axis;

b. determining a defocusing function related to the measuring point based on the light signal intensity of the left section and the right section of the confocal signal related to the measuring point, and performing linear fitting on a curve of the defocusing function;

c. and measuring the object to be measured based on the defocusing function after linear fitting.

8. The method of claim 7,

gradually transitioning the measurement point from a negative defocus state to a positive defocus state while performing a Z-axis step scan on the measurement point, and the confocal signal includes a defocus amount associated with the measurement point and a Z-axis position corresponding to the defocus amount.

9. The method of claim 7, wherein the confocal signal is divided into two portions corresponding to the left and right segments, and the defocus function value is determined using the following equation:

where I1 is the intensity of the optical signal transmitted through the first portion of the detection slit and I2 is the intensity of the optical signal transmitted through the second portion of the detection slit.

10. The method of claim 7, wherein the tilt angle is in the range of 15 ° to 25 °.