Rectangular non-polarizing beam splitter that avoids interference fringes caused by surface reflections

Abstract

A non-polarizing beam splitter can be a rectangular parallelepiped, with the diagonal plane extending at a 45° angle from the corner between the first and second faces to a point on the third or fourth face. A light beam is split into a transmitted beam and a reflected beam using a non-polarizing beam splitter.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to assigned provisional patent application No. 63 / 447,058, filed February 21, 2023, the disclosure of which is incorporated herein by reference.

[0002] This disclosure relates to optical systems, and more particularly to beam splitters. [Background technology]

[0003] Advances in the semiconductor manufacturing industry are increasing the demand for yield management, particularly metrology and inspection systems. As critical dimensions continue to shrink, the industry must reduce the time to achieve high-value production with high yields. Minimizing the total time from detecting a yield problem to correcting it maximizes the return on investment for semiconductor manufacturers.

[0004] Fabricating semiconductor devices, such as logic and memory devices, typically involves processing semiconductor wafers using multiple manufacturing processes to form various features and multiple levels of semiconductor devices. For example, lithography is a semiconductor manufacturing process that involves transferring a pattern from a reticle to a photoresist disposed on a semiconductor wafer. Further examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. An array of multiple semiconductor devices fabricated on a single semiconductor wafer may be separated into individual semiconductor devices.

[0005] Inspection processes are used at various steps during semiconductor manufacturing to detect defects on masks and wafers or other workpieces to promote higher yields and therefore higher profits in the manufacturing process. Inspection has always been an important part of manufacturing semiconductor devices, such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For example, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size becomes necessary because even relatively small defects can cause undesirable aberrations in the semiconductor device.

[0006] Defect review typically involves redetecting defects detected by the inspection process and generating additional information about the defects at higher resolution using either high magnification optics or a scanning electron microscope (SEM). Defect review is typically performed at distinct locations on the specimen where defects were detected by inspection. The high-resolution data of the defects generated by defect review is better suited to determining attributes of the defects, such as profile, roughness, or more precise size information.

[0007] Metrology processes are also used at various steps during semiconductor manufacturing to monitor and control the process. Metrology processes differ from inspection processes in that, unlike inspection processes in which defects are detected on a wafer or other workpiece, metrology processes are used to measure one or more characteristics of the workpiece that cannot be determined using existing inspection tools. Metrology processes may be used to measure one or more characteristics of a workpiece so that the performance of the process can be determined from the one or more characteristics. For example, a metrology process may measure the dimensions (e.g., linewidth, thickness, etc.) of features formed on a workpiece during the process. Additionally, if one or more characteristics of the workpiece are unacceptable (e.g., outside a predetermined range for the characteristics), the measurements of the one or more characteristics of the workpiece may be used to modify one or more parameters of the process so that additional workpieces produced by the process have acceptable characteristics.

[0008] Imaging systems used for inspection or metrology can use beam splitters as part of their optical system. A beam splitter is an optical device that splits a light beam into a transmitted beam and a reflected beam. When the beam splitter is cubic, it is made from two triangular glass prisms glued together at their bases, such as with a polyester, epoxy, or urethane adhesive. The thickness of the resin layer is adjusted so that half of the light incident through one face of the cube (for a specific wavelength) is reflected and the other half is transmitted. A cube beam splitter can have a special coating between the two triangular glass prisms to achieve the designed splitting efficiency between the reflected and transmitted beams, and the two triangular prisms are held together by optical contact only (i.e., no optical adhesive is required). Polarizing beam splitters, such as a Wollaston prism, use birefringent materials to split light into two beams with orthogonal polarization states.

[0009] As shown in Figure 1, a cube beam splitter (BS) can be used in an imaging system to combine and direct the illumination light beam to the sample and the return signal light beam through an image sensor. Depending on its design, such a beam splitter can be polarizing (PBS) or non-polarizing (NPBS). If the diagonal surface S1 in Figure 1 has near-unit reflectance and transmittance for incident s-polarized and p-polarized light, respectively (R s ≒1 and T p ≈1), the cube beam splitter is polarizing. This means that the transmittance and reflectance are near zero for s-polarized and p-polarized light on S1, respectively, i.e., T s ≒0 and R p ≈0. For this situation, the p-polarized component of the return signal beam in Figure 1 passes through a beam splitter to reach the image sensor. Between the beam splitter and the sample are imaging optics (e.g., lenses) and polarization control optics (e.g., wave plates). The diagonal surface S1 is partially reflecting and transmitting (e.g., R ) for s-polarized and p-polarized light, respectively, by a specially designed coating stack on S1. s / T s ~50 / 50 and R p / T p If the beam splitter has a polarization ratio of 1 / 2 s to 1 / 5 s, the cube beam splitter is non-polarizing. In such a case, both the s-polarized and p-polarized components of the return signal beam in Figure 1 can reach the image sensor.

[0010] In the case of a standard cubic non-polarizing beam splitter used in imaging systems, the two reflected beams from its two faces have a nearly zero optical path difference (OPD) between them, and the OPD can be much shorter than the temporal coherence length of the illuminating light. If the two reflective faces have a small angular deviation from their ideal orientation, the two reflected beams will form a small angle, resulting in interference fringes on the imaging system's image sensor. Interference fringes can be bright or dark bands caused by light beams that are in phase or out of phase with each other. Interference fringes can substantially degrade image quality. Reducing or eliminating interference fringes by reducing the small tilt angle (e.g., a few arcsec) deviation of the beam splitter's two reflective faces is difficult because such small angular tolerances approach the limits of manufacturing capabilities or make such cubic beam splitters too expensive.

[0011] Also shown in Figure 1 are two reflections from two surfaces, S2 and S5. The reflectivities of the S2 and S5 anti-reflection (AR) coatings are R2 and R5, respectively. For the s-polarized component in the illumination beam, the fraction of optical power reaching the image sensor from the two reflections is proportional to: R s R5T s +T s R2R s

[0012] For the p-polarized component in the illumination, the fraction of optical power reaching the image sensor from the two reflections is proportional to: R p R5T p +T p R2R p

[0013] The residual reflectances R2 and R5 of the anti-reflection coating are typically small. For polarizing beam splitters, T s ≒0 and R p ≒ 0. However, in the case of a non-polarizing beam splitter, R s , T s , R p , T pSince none of the terms are nearly zero, each of the above four terms is much larger than the polarizing beam splitter term.

[0014] Therefore, for the polarizing beam splitter situation, only a small portion of the light reflections from S2 and S5 reaches the image sensor, but for the non-polarizing beam splitter, unless the reflectivity of the S2 and S5 anti-reflection coatings is close to 0, the portion of the optical power reaching the image sensor from the two surface reflections will be large and may cause imaging problems.

[0015] Figure 2 shows an example of such a situation when using a cube non-polarizing beam splitter. The image contains interference fringes, which are shown by an intensity slice plot. The image in Figure 2 was captured with an inspection system using a cube NPBS. The interference fringes are caused by two reflected beams from cube faces S2 and S5. As shown in Figure 1, three beams reach the image sensor: the signal beam and two reflected beams from S2 and S5. These are shown by the solid, dashed, and dash-dotted lines, respectively. Assuming that there is no interference between the signal and the two reflected beams due to the large OPD between them, and that only the two reflections interfere with each other, the fringe contrast can be calculated using the following equation:

number

[0016] For a signal level of 3%, if R2=0.2% and R5=0.2% (ie, a high quality anti-reflection coating), the calculated fringe contrast is 11.8%.

[0017] Figure 3 illustrates the origin of the observed interference fringes. In Figure 3, surface S2' is the image of surface S2 relative to the diagonal surface S1. Reflection from surface S2 is equivalent to reflection from imaginary surface S2'. In a typical cubic non-polarizing beam splitter, except for surfaces S1 and S2, surfaces S1 and S5, and surfaces S2 and S5 form perfect 45°, 45°, and 90° angles in 3D space, respectively. Surfaces S5 and S2' do not overlap, but the angle between them is small (exaggerated in Figure 3 for ease of understanding). Therefore, the two reflected beams in Figure 3 are not parallel, but form a small angle β' between them outside the beam splitter. Inside the beam splitter, the angle is β = β' / n, where n is the refractive index of the beam splitter material. Using a simple model of two plane wave interference, we can calculate the spatial period Λ of the interference fringes from the optical wavelength λ and the angle β'. Λ=λ / β'

[0018] For λ = 193 nm and β' = 200 μrad or 41.3 arcsec, the fringe period Λ is approximately 1 mm, which corresponds well to the observed fringes on the order of the fringe period. To reduce the effect of the interference fringes, one method is to increase the period of a small-sized image sensor so that its length is less than one interference fringe. If the period is increased to > 10 mm, the angle β' cannot exceed a few arcsec, and the allowable deviation (or tolerance) from their perfect positions of the S2 and S5 planes is further reduced by a factor of the refractive index of the beamsplitter material. Manufacturing such a cubic nonpolarizing beamsplitter with a nearly perfect geometry would be difficult and very expensive. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] U.S. Patent Application Publication No. 2003 / 0086158 [Patent Document 2] European Patent Application Publication No. 2711762 Summary of the Invention [Problem to be solved by the invention]

[0020] Therefore, improved systems and methods are needed. [Means for solving the problem]

[0021] In a first embodiment, a beam splitter is provided. The beam splitter includes a first surface, a second surface connected to the first surface and perpendicular to the first surface, a third surface connected to the first surface and perpendicular to the first surface, and located opposite the second surface, and a fourth surface connected to the second and third surfaces and perpendicular to the second and third surfaces. The first surface has a length different from that of the second surface. The third surface has the same length as the second surface. The fourth surface is located opposite the first surface. A diagonal surface of the beam splitter extends at a 45° angle from the corner between the first and second surfaces to a point on the third or fourth surface. The beam splitter is non-polarizing.

[0022] In one example, the beam splitter is a rectangular parallelepiped.

[0023] The first and fourth surfaces may have the same surface area. The second and third surfaces may have the same surface area.

[0024] The diagonal plane may intersect the third surface at a point between adjacent corners (or edges) of the third surface.

[0025] The diagonal plane may intersect the fourth face at a point between adjacent corners (or edges) of the fourth face.

[0026] The beam splitter may be configured to have no interference fringes due to reflection.

[0027] The optical path length difference of the reflected light at the beam splitter may be longer than the temporal coherence length of the illumination light.

[0028] In a second embodiment, a method is provided. The method includes directing a light beam to a beam splitter. The beam splitter is unpolarized. The beam splitter includes a first surface, a second surface connected to the first surface and perpendicular to the first surface, a third surface connected to the first surface and perpendicular to the first surface and located opposite the second surface, and a fourth surface connected to the second and third surfaces and perpendicular to the second and third surfaces. The first surface has a length different from that of the second surface. The third surface has the same length as the second surface. The fourth surface is located opposite the first surface. A diagonal surface of the beam splitter extends at a 45° angle from the corner between the first and second surfaces to a point on the third or fourth surface. The light beam is split into a transmitted beam and a reflected beam.

[0029] The light beam may be directed to the first surface.

[0030] In one example, the beam splitter is a rectangular parallelepiped.

[0031] The first and fourth surfaces may have the same surface area. The second and third surfaces may have the same surface area.

[0032] The diagonal plane may intersect the third surface at a point between adjacent corners (or edges) of the third surface.

[0033] The diagonal plane may intersect the fourth face at a point between adjacent corners (or edges) of the fourth face.

[0034] The beam splitter may be configured to have no interference fringes due to reflection.

[0035] The optical path length difference of the reflected light at the beam splitter may be longer than the temporal coherence length of the illumination light.

[0036] The method may further include imaging the light beam with an image sensor.

[0037] For a fuller understanding of the nature and objects of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0038] [Figure 1] An example of a beam splitter cube with diagonal face S1 used to combine an illumination beam (solid line) and pass a signal beam (solid line) in an imaging system; the dotted line is the reflected beam from beam splitter face S5, and the dash-dotted line corresponds to the reflection from beam splitter face S2. [Figure 2] An example image (top panel) and intensity slice (bottom panel) with interference fringes along the dashed line in an image from an imaging system using a non-polarizing beam splitter (NPBS) are shown. [Figure 3] The root cause of the observed interference fringes is shown. S2' is the image of surface S2 with respect to surface S1. The tilt between S5 and S2' (exaggerated) creates a non-zero angle between the two reflected beams (dotted and dash-dot lines), which can cause interference fringes on the image sensor. [Figure 4] The relationship between the reflective surface tilt angle and angle β inside the beam splitter is shown. Ray AB is reflected in the directions BD and BC by two surfaces S5 and S2' (not shown) with normal directions BE and BF, respectively. ∠EBF=α is the tilt between the S5 and S2' surfaces. ∠DBC=β is the angle between the two reflections BD and BC. ∠ABE=θ1 and ∠ABF=θ2 are the two angles of incidence. ∠DAC=γ is the angle formed by the two planes ABD and ABC, where AB⊥AC and AB⊥AD. [Figure 5] 1 is an example of a beam splitter according to the present disclosure: A rectangular beam splitter with an elongated surface S2 side to increase the optical path difference (OPD) between the two reflections. [Figure 6] 1 is another example of a beamsplitter according to the present disclosure: a rectangular beamsplitter with an elongated surface S5 side to increase the OPD between the two reflections. [Figure 7] 1 is a flowchart of a method according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0039] Although the claimed subject matter is described with reference to particular embodiments, other embodiments, including embodiments that do not provide all of the advantages and features described herein, are also within the scope of this disclosure. Various structural, logical process step, and electronic changes may be made without departing from the scope of the disclosure. Accordingly, the scope of the present disclosure is defined solely by reference to the appended claims.

[0040] Embodiments disclosed herein include a three-dimensional rectangular non-polarizing beamsplitter design that does not cause interference fringes due to reflections from two of its faces. The novel design increases the OPD between the two reflected beams to be longer than the temporal coherence length of the illumination light, thereby avoiding interference fringes. Design embodiments disclosed herein are more feasible and less expensive to manufacture than other designs that address this issue, such as near-perfect cube beamsplitters with tight manufacturing tolerances. The embodiments disclosed herein avoid the need for near-perfect anti-reflection coatings or the need to further reduce even small tilt deviations between the two faces.

[0041] Figure 4 illustrates the relationship between the inclination angle of the reflecting surfaces and the angle β inside the beam splitter. Ray AB is reflected by two surfaces S5 and S2' (not shown in Figure 4 for clarity) with normal directions BE and BF in the directions BD and BC, respectively. ∠EBF=α is the inclination between the S5 and S2' surfaces. ∠DBC=β is the angle between the two reflections BD and BC. ∠ABE=θ1 and ∠ABF=θ2 are the two angles of incidence on the reflecting surfaces S5 and S2', respectively. ∠DAC=γ is the angle formed by the two planes ABD and ABC, where AB⊥AC and AB⊥AD. For small inclination angles α, the angle between the two reflections is β≈2α, regardless of the angles of incidence (AOI) θ1 and θ2. The general formula for angles α, β, and γ is as follows: cos α=cos(θ1-θ2)+sinθ1sinθ2(cos γ-1) cos β=cos(2θ1-2θ2)+sin2θ1sin2θ2(cos γ-1)

[0042] When θ1 and θ2 are much larger than α, α is small, so γ is also approximately 0. Therefore, cos γ-1 ≒ -γ 2 / 2, the second terms on the right-hand side of the previous two equations are negligible compared to the first terms. As a result, β ≈ 2α. If both θ1 and θ2 are small and comparable to the small angle α, the angle γ does not need to be small. However, the second terms in the equations can be eliminated because the two first terms are close to 1, while the two second terms are nearly zero because the sines of θ1 and θ2, and 2θ1 and 2θ2, are nearly zero. In summary, when the tilt angle α between the S5 and S2' planes is small, the angle β between the two reflected beams is approximately twice α and is independent of the illumination beam incident angle.

[0043] The temporal coherence length of a laser light source is determined by its wavelength and spectral bandwidth. Assuming λ = 193 nm and Δλ = 10 pm, the coherence length is calculated to be 3.7 mm. When a non-polarizing beam splitter is used with such an illumination source, if there is an extra OPD between the two reflections from the S2 and S5 surfaces on the image sensor that is longer than the laser coherence length, no interference fringes will exist between them. The two surface reflections result in simple intensity summation on the image sensor.

[0044] OPD is the difference between two optical path lengths. As shown here, it is the OPD between two surface-reflected beams from S2 and S5, both of which are from the incident beam and can reach the image sensor. The OPD between them is determined by the NPBS and its refractive index at the wavelength of interest.

[0045] 5 and 6 show an embodiment of a beam splitter according to the present disclosure. The beam splitter is three-dimensionally rectangular. In one example, the beam splitter is non-polarizing. 5 and 6 show a light source, an image sensor, and a sample (e.g., a semiconductor mask, wafer, or other workpiece).

[0046] As shown in Figure 5, the beam splitter has a first surface S4 and a second surface S3. The second surface S3 is connected to the first surface S4 and perpendicular to the first surface S4. The second surface S3 is longer than the first surface S4. In this case, connected may mean directly attached or connected to an additional layer or component therebetween.

[0047] The third surface S5 is connected to the first surface S4 and perpendicular to the first surface S4. The third surface S5 is located on the opposite side of the second surface S3. The third surface S5 has the same length as the second surface S3.

[0048] The fourth surface S2 is connected to the second surface S3 and the third surface S5 and is perpendicular to the second surface S3 and the third surface S5. The fourth surface S2 has the same length as the first surface S4.

[0049] The first surface S4, the second surface S3, the third surface S5, and the fourth surface S2 may all be external surfaces, i.e., they may be located outside the beam splitter.

[0050] In FIG. 5 , the beam splitter's diagonal plane S1 extends from the corner between the first surface S4 and the second surface S3 to a point on the third surface S5, which is a non-zero distance from the corner between the third surface S5 and the first surface S4 and a non-zero distance from the corner between the third surface S5 and the fourth surface S2. The diagonal plane S1 extends at a 45° angle with respect to the first surface S4 or the second surface S3. Due to its angle, the diagonal plane S1 does not intersect with the corner between the third surface S5 and the fourth surface S2. Instead, the diagonal plane S1 intersects with the intermediate region between two adjacent corners of the third surface S5.

[0051] As shown in FIG. 6, the beam splitter has a first surface S4 and a second surface S3. The second surface S3 is connected to the first surface S4 and perpendicular to the first surface S4. The first surface S4 is longer than the second surface S3. The third surface S5 is connected to the first surface S4 and perpendicular to the first surface S4. The third surface S5 is located on the opposite side of the second surface S3. The third surface S5 has the same length as the second surface S3. The fourth surface S2 is connected to the second surface S3 and the third surface S5 and perpendicular to the second surface S3 and the third surface S5. The fourth surface S2 is located on the opposite side of the first surface S4. The fourth surface S2 has the same length as the first surface S4.

[0052] In FIG. 6, the diagonal plane S1 of the beam splitter extends from the corner between the first surface S4 and the second surface S3 to a point on the fourth surface S2, which is a non-zero distance from the corner between the fourth surface S2 and the second surface S3 and a non-zero distance from the corner between the fourth surface S2 and the third surface S5. The diagonal plane S1 extends at a 45° angle with respect to the first surface S4 or the second surface S3. Due to its angle, the diagonal plane S1 does not intersect with the corner between the third surface S5 and the fourth surface S2. Instead, the diagonal plane S1 intersects with the intermediate region between two adjacent corners of the fourth surface S2.

[0053] The beam splitter has a rectangular parallelepiped shape (i.e., a three-dimensional rectangle). The beam splitter includes one triangular prism and one polygonal prism that combine to form a rectangular parallelepiped. Although four faces are labeled in Figures 5 and 6, because it is a rectangular parallelepiped, there are two additional faces. These additional faces extend into and out of the page.

[0054] The first surface S4 and the fourth surface S2 have the same surface area. The second surface S3 and the third surface S5 have the same surface area. These surfaces with the same surface area have the same dimensions along their perimeter.

[0055] Using the designs shown in Figures 5 and 6, the beam splitter is configured to have no interference fringes due to reflection. The OPD between the reflected beams at the beam splitter is longer than the temporal coherence length of the illumination light. If the OPD is greater than the temporal coherence length of the illumination light, the occurrence of interference fringes can be zero. For example, the coherence length can be several millimeters to several centimeters. In one example, the coherence length is less than 5 mm. A longer coherence length (e.g., 10 cm) may require a larger non-polarizing beam splitter in one direction.

[0056] In one example, the beam splitter has a refractive index between 1.3 and 1.8. For example, the refractive index may be 1.5. The refractive index varies with wavelength and the material of the beam splitter.

[0057] If the coherence length is <5 mm and the refractive index of the non-polarizing beam splitter material is 1.5, an extra optical path difference of 5 mm will result in a size expansion of the non-polarizing beam splitter at the fourth surface S2 or the third surface S5 of about 1.7 mm. If the coherence length is 50 mm (i.e., 10 times larger), it may still be feasible to increase the non-polarizing beam splitter size by 17 mm at the fourth surface S2 or the third surface S5.

[0058] If the coherence length of the illumination light is not large, the rectangular non-polarizing beam splitter can be designed so that interference fringes do not occur between the two reflected beams from the fourth surface S2 or the third surface S5 after a sufficiently long extra optical path difference is introduced between them. The size expansion can be at either the fourth surface S2 or the third surface S5. The size expansion at the fourth surface S2 is advantageous compared to the third surface S5 because it has no optical effect on the imaging path from the sample to the image sensor. Because S5 is located within the imaging path, the size expansion at S5 directly affects the imaging path.

[0059] FIG. 7 is a flowchart of method 200. A light beam is directed to a non-polarizing beam splitter at 201. The beam splitter may be, for example, as described in FIG. 5, FIG. 6, or another embodiment disclosed herein. Accordingly, the beam splitter may have a rectangular parallelepiped shape. The light beam may be directed to a first surface S4, for example. At 202, the light beam is split into a transmitted beam and a reflected beam. The light beam may be imaged by an image sensor. The beam splitter may be configured to have no interference fringes due to reflection. The optical path length difference of the reflected light at the beam splitter may be longer than the temporal coherence length of the illumination light.

[0060] Both the transmitted beam and the reflected beam cause two surface reflection beams from S2 and S5, respectively. The two surface reflection beams can cause interference fringes on the image sensor. The size expansion of S2 or S5 can increase the OPD between the two surface reflections and eliminate the interference fringes between them.

[0061] Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure. Accordingly, the present disclosure is intended to be limited only by the appended claims and their reasonable interpretation.

Claims

1. A beam splitter comprising: A first surface; a second surface connected to and perpendicular to the first surface, the first surface having a different length than the second surface; a third surface connected to and perpendicular to the first surface and opposite the second surface, the third surface having the same length as the second surface; a fourth surface connected to the second surface and the third surface and perpendicular to the second surface and the third surface, the fourth surface being opposite the first surface; Equipped with a diagonal surface of the beam splitter extends at a 45° angle from a corner between the first surface and the second surface to a point on the third surface or the fourth surface, and the beam splitter is non-polarizing; Beam splitter.

2. The beam splitter of claim 1 , wherein the beam splitter is a rectangular parallelepiped.

3. 2. The beamsplitter of claim 1, wherein the first surface and the fourth surface have the same surface area, and the second surface and the third surface have the same surface area.

4. 2. The beamsplitter of claim 1, wherein the diagonal plane intersects the third surface at a point between adjacent corners of the third surface.

5. 2. The beamsplitter of claim 1, wherein the diagonal plane intersects the fourth surface at a point between adjacent corners of the fourth surface.

6. The beamsplitter of claim 1 , wherein the beamsplitter is configured to have no interference fringes due to reflection.

7. The beam splitter of claim 1 , wherein the optical path length difference between the reflected beams at the beam splitter is longer than the temporal coherence length of the illumination light.

8. directing the light beam to a beam splitter, the beam splitter being non-polarizing, the beam splitter comprising: A first surface; a second surface connected to and perpendicular to the first surface, the first surface having a different length than the second surface; a third surface connected to and perpendicular to the first surface and opposite the second surface, the third surface having the same length as the second surface; a fourth surface connected to the second surface and the third surface and perpendicular to the second surface and the third surface, the fourth surface being opposite the first surface; Equipped with a diagonal surface of the beam splitter extending at a 45° angle from a corner between the first surface and the second surface to a point on the third surface or the fourth surface; Steps and splitting the light beam into a transmitted beam and a reflected beam; A method comprising:

9. The method of claim 8 , wherein the light beam is directed onto the first surface.

10. The method of claim 8 , wherein the beam splitter is a rectangular parallelepiped.

11. The method of claim 8 , wherein the first surface and the fourth surface have the same surface area, and the second surface and the third surface have the same surface area.

12. The method of claim 8 , wherein the diagonal plane intersects the third surface at a point between adjacent corners of the third surface.

13. The method of claim 8 , wherein the diagonal plane intersects the fourth face at a point between adjacent corners of the fourth face.

14. The method of claim 8 , wherein the beam splitter is configured to have no interference fringes due to reflection.

15. The method of claim 8 , wherein the optical path length difference between the reflected beams at the beam splitter is longer than a temporal coherence length of the illumination light.

16. The method of claim 8 further comprising imaging the light beam with an image sensor.

Images