CN1180222C - Dual-frequency confocal step height microscopic measuring device - Google Patents

Abstract

The present invention belongs to the technical field of surface topography measurement and relates to a dual frequency confocal shoulder height micrometering device which comprises a horizontal Zeeman laser, a spectroscope, a Faraday cell, a 1/2 wave plate, a lens, a pinhole, a cemented lens, a polarization spectroscope, a quarter-wave plate which is placed on the reflection light path of the polarization spectroscope, a kaleidoscope prism, a quarter-wave plate which is placed on the transmitted light path of the polarization spectroscope, a microobjective, a second spectroscope, a Glan prism, a convergent lens on the reflection light path of the second spectroscope, a second pinhole and a photodetector on the reflection light path of the second spectroscope, a convergent lens on the transmitted light path of the second spectroscope, and a photodetector on the transmitted light path of the second spectroscope, wherein the spectroscope, the Faraday cell, the 1/2 wave plate, the lens, the pinhole, the cemented lens and the polarization spectroscope are orderly placed on the axial line of the transmitting terminal of the laser; the kaleidoscope prism is placed on the reflection light path of the polarization spectroscope; the microobjective is placed on the transmitted light path of the polarization spectroscope; the Glan prism and the second pinhole are respectively placed on the reflection light path of the second spectroscope. The present invention merges dual frequency laser interference and scanning confocal microtechnique to improve resolution and the measurement range, satisfies the requirements of high measurement precision and a large measuring range and has low cost. The present invention not only satisfies the requirement of the immediate development of microelectronic technology but also can be further popularized and applied.

Description

Dual-frequency confocal step height microscopic measuring device

technical field

The invention belongs to the technical field of surface topography measurement, in particular to a dual-frequency confocal interference microscope system used for step height standard measurement.

Background technique

Confocal scanning imaging was proposed by Minsky as early as the 1950s, and then Davidovits, Sheppard and Wilson made further research on confocal microscopy systems. The research results show that: the confocal scanning microscope system can not only suppress the weak stray light in imaging, but also under the same imaging conditions, its axial resolution is 1.4 times that of the ordinary microscope system, and it has the function of three-dimensional tomography. It is this three-dimensional imaging capability that has made confocal microscopy widely used in the fields of biology, biomedicine, industrial detection, and metrology.

However, the axial resolution of ordinary confocal microscopes is still only at the submicron level, and the noise and drift of the light source directly affect the measurement results. For this reason, Tan Jiubin from Harbin Institute of Technology proposed a differential confocal nanoscale optical Focus detection system, as shown in Figure 1. Its structure comprises: light source 31 and half-mirror 32 and focusing lens 33 that are arranged on its outgoing optical axis, another half-mirror 35 is set on the reflected light path of this half-mirror 32, and set respectively There are two pinholes 36, 38 and two detectors 37, 39 on the reflection and transmission light paths of the half mirror 35, and a processing circuit 40 for receiving the output signals of the two detectors. The working principle of this confocal microscope is: the light output from the light source 31 is reflected back by the surface 34 of the sample to be tested and then divided into two beams of transmitted and reflected light by the beam splitter 35, and pinholes and detectors are respectively placed after the reflected light and the transmitted light. The positions of the pinholes 36 and 38 are symmetrically located in front of and behind the image focal plane respectively, and the optical focus signal is given by taking the difference and sum of the signals 37 and 39 of the two detectors. When the surface of the sample to be measured is on the focal plane, the positions of the two pinholes are relatively symmetrical to the focal plane of the image, and the difference between the two detectors is zero; when the surface of the sample deviates from the focal plane by a small displacement, the image points approach respectively One of the pinholes is far away from the other pinhole, so that the detected optical power increases and decreases, so that the magnitude and direction of the displacement are reflected by the differential signal. Although this method can make the axial resolution of the measurement reach 2nm, the measurement range is limited by the linear region of the light intensity differential curve, and cannot provide a larger range of measurement. At the same time, since the relationship between the change of light intensity and the displacement of the sample surface is not completely linear, the measurement accuracy cannot be very high.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, integrate the characteristics of dual-frequency laser interference and scanning confocal microscopy technology, and provide a dual-frequency confocal microscopic step height measurer device, which can measure the steps of microelectronic mask boards, etc. Quantitative measurement of height, with the characteristics of high resolution and large range measurement.

The technical scheme of the present invention includes a transverse Zeeman dual-frequency laser, a first beam splitter, a Faraday cell, a 1/2 wave plate placed at 22.5 degrees with the fast axis, a lens, a first Pinhole, cemented lens and polarization beam splitter; the first 1/4 wave plate and tetrahedral prism placed on the reflection light path of the polarization beam splitter with the fast axis as 45 degrees, placed on the transmission light path of the polarization beam splitter The second 1/4 wave plate and the microscope objective lens placed at 45 degrees with the fast axis; also include a second beam splitter arranged on the combined light path of the polarization beam splitter, respectively placed on the second beam splitter A Glan prism, a converging lens, a second pinhole and a photodetector on the reflected light path, and a converging lens and a photodetector on the transmitted light path of the second beam splitter.

The measurement principle of the present invention is as follows: as shown in FIG. 2 . The transverse Zeeman (He-Ne dual-frequency) laser is used as the light source of the confocal microscope and the error interference system at the same time to obtain the light intensity information (coarse positioning) and phase information (precise value) of the sample surface respectively, and at the same time realize a large range and High resolution step height measurement requirements.

Assume that the polarization vector of the laser output from the transverse Zeeman laser is:

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 01</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 01</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; j</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 02</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 02</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

分别为出射平行分量p、垂直分量s的偏振方向矢量；E₀₁、E₀₂分别为p、s分量的振幅；f₁、f₂分别为p、s分量的频率，频差Δf＝f₁-f₂；φ₀₁、φ₀₂分别为p、s分量的初始相位。In the formula, the meaning of each variable is:

are the polarization direction vectors of outgoing parallel component p and vertical component s respectively; E ₀₁ and E ₀₂ are the amplitudes of p and s components respectively; f ₁ and f ₂ are the frequencies of p and s components respectively, and the frequency difference Δf=f ₁ - f ₂ ; φ ₀₁ and φ ₀₂ are the initial phases of the p and s components respectively.

After E ₀ is split by the PBS, the reflected light is the s component to form the reference light, and the transmitted light is the p component to form the measurement light. After passing through the reference and measurement optical paths respectively, the light is combined by the PBS. Since the reference light passes through the 1/4 wave plate twice, which is equivalent to passing through the 1/2 wave plate once, and because the fast axis of the 1/4 wave plate is 45° to the polarization direction, the polarization components of the reference light are rotated by 90° respectively. °, that is, from s-polarization to p-polarization; similarly, the measurement light changes from p-polarization to s-polarization. Therefore, when the reference light passes through the PBS when it returns from the tetrahedron, the measurement light is reflected by the PBS when it is reflected from the surface of the sample to be tested, and the two are recombined into a beam of light. The polarization vectors E ₁ and E ₂ of the reference light and measurement light after PBS light combination can be written as:

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="K"\; filenames="C0212088400091.png"\; state="\{\{state\}\}">K</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 02</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 02</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; j</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="K"\; filenames="C0212088400091.png"\; state="\{\{state\}\}">K</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 01</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="f"\; filenames="C0212088400091.png"\; state="\{\{state\}\}">f</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 01</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; twenty\; two</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Among them, K ₁ =1-α ₁ , K ₂ =1-α ₂ , α ₁ , α ₂ are the light intensity loss factors of the s and p components in the interferometer respectively; φ ₁₂ is the phase increment of the reference optical path, φ ₂₂ To measure the phase increment of the optical path when the samples are different. F ₀ is the Doppler frequency shift caused by the sample scanning process.

After E ₁ and E ₂ are depolarized, they are received by the detector to form measurement signals E _m and I _m , which can be described as:

I _m ＝I ₀ K ₁ K ₂ sin(2π(f ₁ -f ₂ +2f ₀ )t+φ ₀₁ +φ ₂₂ -(φ ₀₂ +φ ₁₂ )) (4) where: I ₀ ∝ E ₀₁ E ₀₂ .

The reference signal I _r taken directly from the output of the transverse Zeeman laser is:

I _r ＝I ₀ K ₀ sin(2π(f ₁ -f ₂ )t+φ ₀ ) (5)

Since φ ₀₁ , φ ₂₂ , φ ₀₂ , φ ₁₂ , and φ ₀ are all constants, the phase difference Δφ obtained after comparing I _m with I _r is:

In the formula, λ=632.8nm, which is the wavelength of He-Ne laser; s is the measured displacement; the unit of Δφ is degree. From formula (6) can get: $\mathrm{the\; s}=\frac{\mathrm{\&Delta;\&phi;}}{720}\&CenterDot;\mathrm{\&lambda;},$ So when the phase measurement resolution is 0.1°, the displacement measurement resolution reaches 0.1nm.

However, the phase measurement method is only valid within half a wavelength range. Therefore, in order to increase the measurement range, the present invention uses a confocal microscope system to measure the light intensity synchronously, thus achieving high resolution of step height measurement while meeting a larger measurement range.

The present invention has following characteristics and good effect:

The dual-frequency confocal microscopic step height measuring device of the present invention adopts a dual-frequency laser as a light source on the basis of light intensity measurement by a traditional confocal microscope, and adds phase detection, thereby breaking through the limitation of the Rayleigh criterion for light intensity detection. The phase measurement method realizes 3600 subdivisions of the half-wavelength (equivalent to a phase measurement unit of 0.1°) so that the measurement resolution reaches the nanometer level. The improved optical path of the dual-frequency confocal interference microscope system is one of the innovative points of the present invention which are different from the prior art.

The confocal microscopic measurement technology of the present invention adopts Faraday cell and 1/2 wave plate, makes outgoing light and returning light polarization direction (being s polarization and p polarization) exchange, thereby effectively overcomes the influence of light feedback problem, makes laser The frequency stabilization system can always work normally during the measurement process, avoiding multiple resonant cavities.

The present invention obtains rough positioning information by light intensity measurement while achieving high resolution, and the light intensity signal measurement is given by sinusoidal signal amplitude obtained by double-frequency interference. The measurement signal and the reference signal of the same frequency are input into the lock-in amplifier, and the amplitude of the measurement signal can be obtained. Compared with the measurement of DC light intensity by ordinary confocal microscope, this method can effectively overcome the interference of extraneous stray light and improve the robustness of measurement. This is the second innovative point that distinguishes the prior art.

Description of drawings

Figure 1 is the optical path structure diagram of the existing differential confocal nanoscale optical focusing detection system

Fig. 2 is the measuring principle block diagram of the present invention

Fig. 3 is the structural diagram of an embodiment of the dual-frequency confocal step height microscopic measurement device of the present invention

Fig. 4 is the implementation example of the present invention and nano-interferometer principle block diagram

Fig. 5 is a group of preliminary experimental results of the embodiment of the present invention

Detailed ways

The structure and working principle of the dual-frequency confocal step height microscopic measurement device of the present invention are described in detail in conjunction with the embodiments and accompanying drawings as follows:

The structure of the present embodiment is shown in Figure 3, including: a transverse Zeeman He-Ne dual-frequency laser 1 with higher frequency stabilization accuracy, and a beam splitter (BS) 2 and a Faraday cell placed on the axis of the laser emitting end in sequence 4, 1/2 wave plate 5, lens 6, pinhole 7, cemented lens 8 and polarization beam splitter (PBS) 9, place 1/4 wave plate 10 and tetrahedral prism 11 on PBS reflection light path, place in PBS The 1/4 wave plate 12 and the microscope objective lens 13 on the transmitted light path; the measurement light and the reference light are combined by PBS and then divided into two parts for light intensity measurement and phase measurement respectively, so this device also includes BS15, which are placed separately Glan prism 19, cemented lens 20, pinhole 21 and photodetector 22 on the BS reflected light path, diaphragm 16, converging lens 17 and photodetector 18 on the BS15 transmitted light path.

The specific principles of the dual-frequency confocal step micro-height measuring device in this embodiment are as follows:

As shown in Figure 3. Transverse Zeeman He-Ne laser 1 outputs orthogonal dual-frequency laser, and BS 2 divides the output light into two beams. After the reflected light is analyzed, the detector 3 forms a reference signal, and the transmitted light passes through the Faraday cell 4 and along the fast axis. The 1/2 wave plate 5 placed in the direction of 22.5° rotates the polarization direction of light by 90°, which can avoid the influence of light feedback on the laser. Then converge on the pinhole 7 through the small lens 6, filter the stray light through the pinhole 7, expand the light beam into a parallel light beam by the cemented lens 8. The parallel light beam is polarized and split through PBS9, in which the reflected light is used as the reference light, and the transmitted light is used as the measuring light. The reference light first passes through the 1/4 wave plate 10 placed along the fast axis at 45°, and then is reflected by the tetrahedron 11. The reflected light passes through the 1/4 wave plate 10 placed at 45° again, and the polarization direction of the reference light is rotated by 90°. When PBS9, it becomes transmitted light; the measurement light passes through the 1/4 wave plate 12 placed along the fast axis at 45° and the microscopic objective lens 13 with infinite tube length and projects on the surface 14 of the sample to be measured, and the measurement light passes through 1 again after being reflected by the sample. After the /4 wave plate 12, the polarization direction is also rotated by 90°, and after passing through the PBS9 again, it is reflected and combined with the reference light. The synthesized light is divided into two parts by BS15, and the transmitted light is analyzed and received by the detector 18 after passing through the diaphragm 16 and the converging lens 17 to form a phase measurement signal, and the phase measurement is carried out with the reference signal, so that the resolution of the dual-frequency confocal microscope is increased to Below 1 nanometer; the reflected light passes through the Glan prism 19 to filter out the reference light, and after being converged by the cemented lens 20 (the same as the cemented lens 8) through the pinhole 21, it is incident on the detector 22 to receive, forming a light intensity measurement signal, which passes through the lock The phase amplifier obtains the light intensity value, and the information of the axial displacement deviating from the focal point is obtained from the light intensity change, so as to overcome the single value problem of pure phase measurement, make the longitudinal measurement range reach more than 5 microns, and effectively reduce stray light interference at the same time.

The method for measuring the height of the microelectronic step in this embodiment includes the following steps:

1) The transverse Zeeman He-Ne laser 1 outputs an orthogonal dual-frequency laser, and the output light is divided into two beams by BS2. After the reflected light is analyzed, the reference signal is formed by the detector 3, and the transmitted light passes through the Faraday cell 4 and the fast axis. The 1/2 wave plate 5 placed along the 22.5° direction rotates the polarization direction by 90° to avoid the influence of light feedback.

2) Converge on the pinhole 7 through the small lens 6, filter the stray light through the pinhole 7, and expand the beam into a parallel beam by the cemented lens 8.

3) The parallel light beam is polarized and split through PBS9, wherein the reflected light is used as the reference light, and the transmitted light is used as the measuring light.

4) The reference light first passes through the 1/4 wave plate 10 placed along the fast axis at 45°, and then is reflected by the tetrahedron 11. The reflected light passes through the 1/4 wave plate 10 placed at 45° again, and the polarization direction of the reference light is rotated by 90°. It becomes transmitted light when passing through PBS9 again.

5) The measurement light is projected on the surface 14 of the sample to be tested through the 1/4 wave plate 12 placed along the fast axis at 45° and the microscope objective lens 13 with infinite tube length, the measurement light is reflected back by the sample, and after passing through the microscope objective lens 13, it is again After passing through the 1/4 wave plate 12, its polarization direction is also rotated by 90°, and after reaching the PBS9, it is reflected and combined with the reference light.

6) The synthesized light is divided into two parts by the BS15, and the transmitted light is analyzed and received by the detector 18 after passing through the diaphragm 16 and the converging lens 17 to form a phase measurement signal, and the phase measurement is performed with the reference signal, so that the resolution of the dual-frequency confocal microscope increased to less than 1 nm.

7) The reflected light passes through the Glan prism 19 to filter out the reference light, and after being converged by the cemented lens 20 (the same as the cemented lens 8) through the pinhole 21, it is incident on the detector 22 for reception, and the measurement signal is obtained through a lock-in amplifier to obtain the light intensity value , the information of the axial displacement away from the focal point is obtained from the change of the light intensity, so as to overcome the single value problem of the pure phase measurement, make the longitudinal measurement range reach more than 5 microns, and effectively reduce the stray light interference.

In this method, the confocal microscopic technique and the heterodyne interferometry technique are adopted, and the requirements of a large measuring range and a high measuring resolution are met at the same time. The light intensity measurement uses the phase-locking principle, which overcomes the problem that purely measuring DC light intensity is easily affected by stray light. The phase measurement of the heterodyne interference signal is realized by using the FPGA phase card, and the resolution can reach 0.1°, corresponding to a resolution of about 0.1 nanometer. Light intensity measurements and phase measurements are received separately by a computer.

Models and parameters of the main components of embodiments of the present invention:

Transverse Zeeman laser 1 frequency difference is tens to hundreds of kilohertz, the transmittance of BS2 is 90%, the reflectivity is 10%, the detectors 3, 18, 22 are PIN tubes, and the Faraday box 4 rotates the polarization direction by 45 °, the angle between the fast axis direction of the 1/2 wave plate 5 and the horizontal direction is 22.5°, the focal length of the cemented lens 8 is 150mm, the angle between the 1/4 wave plate 10, 12 and the horizontal direction is 45°, BS15 transmission and reflection The ratio is 1:1.

In order to calibrate the measurement accuracy of the system in this embodiment, the differential nano-interferometer SJD5 is used to calibrate the system, and the method is shown in Figure 4. The micro-displacement workbench is driven by the piezoelectric ceramic PZT, and the confocal micro-interference system of the present invention and the differential nano-interferometer SJD5 (this interferometer is calibrated by the Institute of Metrology, the measurement resolution is sub-nanometer, and the nonlinear error is 2.8 nanometers) are used for measurement. , the computer is used for measurement synchronization control, and the results are also collected by the computer. Figure 5 shows a set of experimental curves. The abscissa is the measurement result of SJD5 when the PZT drives the worktable, the left side of the ordinate is the phase measurement result of the confocal micro-interference system, and the right side of the ordinate is the light intensity measurement result. The experimental results show that the phase measurement The nonlinear error is not more than 7 nm.

Claims (1)

1. A dual-frequency confocal step height microscopic measurement device is characterized in that it comprises a transverse Zeeman dual-frequency laser, and is placed on the first beam splitter, Faraday cell, and fast axis on the axis of the laser emitting end successively. 1/2 wave plate, lens, first pinhole, cemented lens and polarizing beam splitter placed at 22.5 degrees; the first 1/4 wave plate and the first 1/4 wave plate placed on the reflection light path of the polarizing beam splitter with the fast axis as 45 degrees A tetrahedral prism, a second 1/4 wave plate and a microscopic objective lens placed on the transmission light path of the polarization beam splitter with the fast axis at 45 degrees; A beamsplitter, a Glan prism, a converging lens, a second pinhole and a photodetector placed on the reflected light path of the second beamsplitter, a converging lens and a photoelectric detector on the transmitted light path of the second beamsplitter detector.

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