Laser processing equipment

Abstract

A laser processing device (1) comprises: a variable focal length optical system (10) which periodically changes a focus position (Pf) according to an input drive signal (Cf); a position detection light source (41) which irradiates a workpiece (W) with detection light via the variable focal length optical system (10); a light detector (44) which receives the detection light (Lm) reflected by the workpiece (W) and outputs a light detection signal (Int); a signal processing unit (62) which, based on the input light detection signal (Int), outputs a synchronization pulse signal (Sync) synchronized with the focus timing of the detection light on the surface of the workpiece (W); and a laser oscillator (311) which oscillates a pulsed laser (Lp) based on the input synchronization pulse signal (Sync) and irradiates the pulsed laser (Lp) onto the workpiece (W) via the variable focal length optical system (10).

Description

Technical Field

[0001] The present invention relates to a laser processing device using a variable focal length optical system. Background Art

[0002] Conventionally, in the manufacturing fields of FPD (Flat Panel Display) and IC (Integrated Circuit) wafers, laser processing devices are known that perform laser processing on a workpiece while observing it for the purpose of correcting defects in fine circuits (e.g., see Document 1: Japanese Patent Application Publication No. 2012-55910). In such laser processing devices, an optical system such as an objective lens is shared between an observation optical system and a laser processing optical system, and a laser oscillator irradiates a workpiece positioned within the field of view of the observation optical system with a laser processing mask disposed within the laser processing optical system.

[0003] In addition, it is known in the past that there is an image detection technology that uses a variable focal length optical system in which the focus position changes periodically (for example, refer to document 2: Japanese Patent Gazette No. 2018-84821). In this image detection technology, a focus scan image is obtained by shooting while periodically changing the focus position relative to the object. The image is a mixed image in which the focus state and the defocus state are mixed. By performing deconvolution processing on the mixed image, an EDOF image (Extended Depth Of Focus image, extended focus depth image) is generated. In the EDOF image, since an effective depth of focus of several to dozens of times the usual depth of focus is obtained, the frequency of refocusing actions that must be performed due to tilt and step differences of the workpiece can be reduced.

[0004] The inventors of the present invention are studying the application of the above-mentioned variable focal length optical system to the laser processing apparatus described in Document 1 and the like.

[0005] However, when a variable focal length optical system is incorporated into a laser processing optical system, the focal position of the laser processing optical system relative to the workpiece changes, making it difficult to accurately focus the laser light on a desired location on the workpiece surface, and reducing the accuracy of laser processing. Summary of the Invention

[0006] An object of the present invention is to provide a laser processing apparatus capable of performing laser processing with high precision while using a variable focal length optical system.

[0007] The laser processing device of the present invention comprises: a variable-focal-length optical system, which periodically changes the focus position according to an input drive signal; a light source, which irradiates detection light to the workpiece via the variable-focal-length optical system; a light detector, which receives the detection light reflected on the workpiece and outputs a light detection signal; a signal processing unit, which outputs a synchronization pulse signal synchronized with the focus timing of the detection light focusing on the surface of the workpiece based on the input light detection signal; and a laser oscillator, which oscillates a pulsed laser based on the input synchronization pulse signal and irradiates the pulsed laser to the workpiece via the variable-focal-length optical system.

[0008] Preferably, the laser processing device of the present invention also includes a laser processing mask, which is arranged between the variable focal length optical system and the laser oscillator at a position that forms a conjugate relationship with the focusing position of the variable focal length optical system, and forms an opening portion through which the pulsed laser passes.

[0009] Preferably, in the laser processing device of the present invention, the light detector is configured so that when the focusing position of the variable focal length optical system is consistent with the surface of the workpiece, the light detection signal becomes a peak value, and the signal processing unit uses the peak value of the light detection signal as the focus timing detection and outputs the synchronization pulse signal synchronized with the focus timing.

[0010] Preferably, the laser processing device of the present invention also includes a photographing element for photographing the workpiece through the variable focal length optical system, and the variable focal length optical system has: a liquid resonance lens, whose refractive index changes periodically according to the input drive signal; an objective lens, which is arranged on the same optical axis as the liquid resonance lens; and a plurality of relay lenses, which are arranged in a manner such that the exit pupil of the objective lens is conjugate with the principal point position of the liquid resonance lens. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a schematic diagram showing a laser processing apparatus according to one embodiment of the present invention;

[0012] Figure 2 is a block diagram showing the laser processing apparatus of the above embodiment;

[0013] Figure 3 It is a graph for explaining the operation of the laser processing apparatus according to the above embodiment. DETAILED DESCRIPTION

[0014] An embodiment of the present invention will be described with reference to the accompanying drawings.

[0015] like Figure 1As shown, a laser processing apparatus 1 includes an observation optical system 2 for observing a workpiece W, a laser processing optical system 3 for laser processing the surface of the workpiece W, and a timing detection optical system 4 for detecting the timing of the laser processing. These optical systems share a variable focus optical system 10 having a liquid resonant lens 12. The surface of the workpiece W is arranged to intersect an optical axis A passing through the variable focus optical system 10.

[0016] In addition, the laser processing apparatus 1 further includes a lens control unit 5 for controlling the operation of the liquid resonance lens 12 and a control unit 6 (see Figure 2 ).

[0017] (Variable focal length optical system)

[0018] First, the variable focus optical system 10 will be described.

[0019] like Figure 1 As shown, the variable focal length optical system 10 includes an objective lens 11 , a liquid resonance lens 12 , and relay lenses 13 and 14 , which are arranged on an optical axis A.

[0020] The objective lens 11 is composed of a conventional convex lens or a lens group.

[0021] The liquid resonant lens 12 comprises a cylindrical housing filled with a liquid such as silicone and a cylindrical vibrating element formed from a piezoelectric material. The vibrating element, immersed in the liquid within the housing, is connected to an external lens control unit 5 via a signal line and vibrates in response to a drive signal Cf input from the lens control unit 5. The drive signal Cf is, for example, a sinusoidal AC signal. When the frequency of the drive signal Cf is adjusted to the resonant frequency, a standing wave is generated in the liquid within the liquid resonant lens 12, causing the refractive index of the liquid resonant lens 12 to change periodically.

[0022] The focus position Pf of the variable focal length optical system 10 on the workpiece W changes periodically based on the focal length of the objective lens 11 and along with the change in the refractive index of the liquid resonance lens 12 .

[0023] The relay lenses 13 and 14 are arranged so that the exit pupil of the objective lens 11 is conjugate with the principal point position of the liquid resonance lens 12, and an aperture (not shown) is arranged at the intermediate image position between the relay lenses 13 and 14. Since the relay lenses 13 and 14 relay the exit pupil of the objective lens 11 while maintaining the telecentric optical system, the magnification of the image incident on the image sensor (described later) remains constant even if the focus position Pf changes.

[0024] (Photographic optical system)

[0025] Next, the observation optical system 2 will be described.

[0026] The observation optical system 2 includes, in addition to the aforementioned variable focal length optical system 10 , an observation illumination light source 21 , a light guide 22 , an illumination optical system 23 , beam splitters 24 to 26 , an imaging lens 27 , and a camera 28 .

[0027] The observation illumination light source 21 is composed of a light emitting element such as an LED, and emits illumination light Li as continuous light.

[0028] The light guide 22 is composed of an optical fiber or the like, and transmits the illumination light Li emitted from the observation illumination light source 21 to the illumination optical system 23 .

[0029] The illumination optical system 23 includes a collector lens 231 and a condenser lens 232 , and appropriately adjusts the illumination light Li transmitted from the light guide 22 .

[0030] The beam splitter 24 is disposed on the image side of the objective lens 11 on the optical axis A, and guides the illumination light Li regulated by the illumination optical system 23 to the objective lens 11 . The illumination light Li is irradiated onto the workpiece W via the objective lens 11 .

[0031] The beam splitter 25 is arranged on the image side of the liquid resonance lens 12 on the optical axis A. The beam splitter 25 reflects the reflected light Lr reflected on the surface of the workpiece W and passing through the focal length variable optical system 10 , and guides the reflected light Lr to the beam splitter 26 .

[0032] It should be noted that the beam splitters 24 and 25 transmit light along the optical axis A of the variable focus optical system 10 .

[0033] The beam splitter 26 reflects the reflected light Lr reflected by the beam splitter 25 and guides it to the imaging lens 27. The beam splitter 26 has wavelength selectivity and reflects the reflected light Lr from the illumination light Li while transmitting the detection light Lm described later.

[0034] The imaging lens 27 constitutes an infinity correction optical system together with the relay lens 14 of the variable focal length optical system 10 . The imaging lens 27 forms an image of the reflected light Lr via the beam splitters 25 and 26 on an imaging element 281 to be described later.

[0035] The camera 28 includes an imaging element 281 such as a CCD (Charge Coupled Device) image sensor, and outputs an image Im captured by the imaging element 281 to the image processing unit 63 in a predetermined signal format. The frame rate of the camera 28 is set to be lower than the period of the drive signal Cf input to the liquid resonance lens 12. Therefore, due to the relationship between the period of change in the focus position Pf and the frame rate of the camera 28, the image Im is a mixed image of a focused state and a defocused state.

[0036] (Laser processing optical system)

[0037] Next, the laser processing optical system 3 will be described.

[0038] The laser processing optical system 3 includes a laser head 31 and a laser imaging lens 32 in addition to the variable focal length optical system 10 .

[0039] The laser head 31 includes a laser oscillator 311 and a laser processing mask 312 .

[0040] The laser oscillator 311 oscillates pulsed laser light Lp having a wavelength suitable for the purpose of processing the workpiece W. The laser oscillator 311 controls the timing of oscillating the pulsed laser light Lp using a synchronization pulse signal Sync described later.

[0041] The laser processing mask 312 has an opening 312A having a predetermined shape such as a circle, and limits the diameter of the pulsed laser light Lp from the laser oscillator 311 to a desired size.

[0042] The laser imaging lens 32 is disposed between the variable focus optical system 10 and the laser head 31 on the optical axis A, and constitutes an infinity correction optical system together with the relay lens 14 of the variable focus optical system 10 .

[0043] Here, the laser processing mask 312 is arranged at the rear focus of the laser imaging lens 32. That is, the laser processing mask 312 is arranged at a position on the optical axis A that is conjugate to the focus position Pf of the variable focal length optical system 10.

[0044] The behavior of the pulsed laser light Lp in the laser processing optical system 3 will be described later.

[0045] (Timing detection optical system)

[0046] Next, the timing detection optical system 4 will be described.

[0047] The timing detection optical system 4 is an optical system for detecting the timing when the laser oscillator 311 oscillates the pulsed laser light Lp, and includes a position detection light source 41 , a light guide 42 , a collimating lens 43 , and a photodetector 44 in addition to the aforementioned variable focal length optical system 10 .

[0048] The position detection light source 41 is, for example, a laser light source, and emits detection light Lm as continuous light.

[0049] The light guide 42 includes an optical fiber splitter 421 and optical fibers 422 to 424. The optical fiber splitter 421 has an optical path for connecting one end of each of the optical fibers 422 to 424. The optical fiber splitter 421 guides light incident from the optical fiber 422 to the optical fiber 423 and guides light incident from the optical fiber 423 to the optical fiber 424.

[0050] The other end of the optical fiber 422 is connected to the position detection light source 41. Therefore, the detection light Lm emitted from the position detection light source 41 propagates through the light guide 42 and is emitted from the end face 423e at the other end of the optical fiber 423. In other words, the end face 423e of the optical fiber 423 functions as a point light source of the detection light Lm.

[0051] The other end of the optical fiber 424 is connected to the photodetector 44 . Therefore, the detection light Lm incident on the end face 423 e of the optical fiber 423 propagates through the light guide 42 and is incident on the photodetector 44 .

[0052] Here, the end face 423e of the optical fiber 423 is arranged at the rear focus of the collimator lens 43. That is, the end face 423e of the optical fiber 423 is arranged at a position on the optical axis A that is conjugate to the focus position Pf of the variable focal length optical system 10.

[0053] The collimator lens 43 collimates the detection light Lm emitted from the end face 423e of the optical fiber 423. The detection light Lm collimated by the collimator lens 43 passes through the beam splitter 26 and is reflected by the beam splitter 25, thereby irradiating the workpiece W via the variable focus optical system 10.

[0054] Furthermore, the collimator lens 43 condenses the detection light Lm that is reflected on the surface of the workpiece W and passes through the variable focal length optical system 10 .

[0055] The photodetector 44 is, for example, a photomultiplier tube or a photodiode, and is connected to the other end of the optical fiber 424. The photodetector 44 receives the detection light Lm incident through the optical fiber 424 and outputs a light detection signal Int corresponding to the intensity of the received light.

[0056] In the timing detection optical system 4, as described above, the focus position Pf of the variable focal length optical system 10 changes periodically. Therefore, only when the focus position Pf coincides with the surface of the workpiece W does the detection light Lm reflected from the surface form a light spot at the rear focus of the collimating lens 43 and enter the end face 423e of the optical fiber 423.

[0057] Therefore, the detection light Lm incident on the photodetector 44 reaches a maximum when the focus position Pf coincides with the surface of the workpiece W. That is, when the focus position Pf coincides with the surface of the workpiece W, the light detection signal Int output by the photodetector 44 shows a peak value.

[0058] (Control Department)

[0059] Figure 2 The control unit 6 shown is composed of, for example, a computer having a CPU (Central Processing Unit) and memory. The control unit 6 implements its intended functions by executing prescribed software. The control unit 6 includes a lens setting unit 61 for setting the lens control unit 5, a signal processing unit 62 for processing various input signals, and an image processing unit 63.

[0060] The lens setting unit 61 sets the frequency, amplitude, maximum drive voltage, and the like of the drive signal Cf for the lens control unit 5 .

[0061] It should be noted that the resonant frequency of the liquid resonance lens 12 changes according to changes in ambient temperature, etc. Therefore, the lens setting unit 61 changes the frequency of the drive signal Cf in real time through feedback control to achieve stable operation of the liquid resonance lens 12 .

[0062] The signal processing unit 62 outputs the synchronization pulse signal Sync to the laser oscillator 311. The signal processing unit 62 switches the level of the synchronization pulse signal Sync based on the input light detection signal Int.

[0063] The image processing unit 63 generates an EDOF image by performing a deconvolution process on the image Im input from the camera 28. It should be noted that the deconvolution process is a process that performs an inverse calculation on the out-of-focus component estimated from the variable range of the focus position Pf with respect to the image Im, thereby generating an EDOF image that is focused on almost the entire variable range of the focus position Pf.

[0064] (Laser processing action)

[0065] Next, the laser processing operation of this embodiment will be described.

[0066] like Figure 3As shown, during the operation of the liquid resonance lens 12, the focus position Pf of the variable focal length optical system 10 changes periodically in synchronization with the drive signal Cf. Figure 3 The surface position of the workpiece W (workpiece position Pw) in the variation range of the focus position Pf on the optical axis A is illustrated.

[0067] The light detection signal Int shows a peak at the timing (focus timing T) when the focus position Pf coincides with the workpiece position Pw, and shows a peak twice per cycle of the drive signal Cf.

[0068] The synchronization pulse signal Sync is at a high level while the light detection signal Int is equal to or higher than the threshold value Vt, and is at a low level while the light detection signal Int is lower than the threshold value Vt.

[0069] The laser oscillator 311 oscillates pulsed laser light Lp at the timing when the input light detection signal Int switches from a low level to a high level. The output period of one pulsed laser light Lp is set to be sufficiently short relative to the change cycle of the focus position Pf, and the threshold value Vt of the light detection signal Int is set so that the output period of the pulsed laser light Lp coincides with the focus timing T. Therefore, the laser oscillator 311 can emit pulsed laser light Lp in sync with the focus timing T.

[0070] Reference again Figure 1 The pulse laser Lp emitted from the laser oscillator 311 at the focus timing T passes through the opening 312A of the laser processing mask 312 and is irradiated onto the workpiece W via the laser imaging lens 32 and the variable focal length optical system 10 .

[0071] Here, the laser processing mask 312 is arranged at a position that forms a conjugate relationship with the focus position Pf of the variable focal length optical system 10 .

[0072] Therefore, the shape of the pulsed laser light Lp that has passed through the laser processing mask 312 is imaged on the focus position Pf of the variable focal length optical system 10 at the focus timing T, that is, on the surface of the workpiece W.

[0073] 〔Effect〕

[0074] As described above, the laser processing device 1 of this embodiment has: a variable-focal-length optical system 10, which periodically changes the focus position Pf according to the input drive signal Cf; a position detection light source 41, which irradiates the detection light Lm to the workpiece W via the variable-focal-length optical system 10; a light detector 44, which receives the detection light Lm reflected on the workpiece W and outputs a light detection signal Int; a signal processing unit 62, which outputs a synchronization pulse signal Sync synchronized with the focus timing T of the detection light Lm focusing on the surface of the workpiece W based on the input light detection signal Int; a laser oscillator 311, which oscillates the pulsed laser Lp based on the input synchronization pulse signal Sync, and irradiates the pulsed laser Lp to the workpiece W via the variable-focal-length optical system 10.

[0075] In such a structure, the detection light Lm emitted from the position detection light source 41 passes through the variable focal length optical system 10 and is irradiated onto the workpiece W while changing the focus position Pf in the optical axis direction. Since the detection light Lm reflected on the workpiece W is received by the light detector 44 while being affected by the change in the focus position Pf, the light detection signal Int input from the light detector 44 includes the influence of the change in the focus position Pf. Based on such a light detection signal Int, the signal processing unit 62 can detect the timing (focus timing T) when the focus position Pf of the detection light Lm coincides with the surface of the workpiece W, and output a synchronization pulse signal Sync synchronized with the focus timing T. In addition, the laser oscillator 311 oscillates the pulsed laser Lp based on the synchronization pulse signal Sync input from the signal processing unit 62, thereby being able to emit the pulsed laser Lp in coordination with the focus timing T.

[0076] Therefore, in the laser processing apparatus 1 of this embodiment, the pulsed laser light Lp irradiated onto the workpiece W via the variable focus optical system 10 is focused on the surface of the workpiece W. Therefore, the pulsed laser light Lp can be accurately focused on a desired portion of the surface of the workpiece W. Thus, high-precision laser processing can be performed while utilizing the variable focus optical system 10.

[0077] The laser processing apparatus 1 of this embodiment also includes a laser processing mask 312 having an opening 312A formed therein, through which the pulsed laser light Lp passes. Since the laser processing mask 312 is disposed between the variable focus optical system 10 and the laser oscillator 311 at a position conjugate to the focus position Pf of the variable focus optical system 10, the shape of the opening 312A is well imaged on the surface of the workpiece W, and an area conforming to the shape of the opening 312A is laser processed.

[0078] Furthermore, in the laser processing apparatus 1 of this embodiment, the photodetector 44 is configured so that the light detection signal Int reaches a peak value when the focus position Pf of the variable-focus optical system 10 coincides with the surface of the workpiece W. The signal processing unit 62 detects the peak value of the light detection signal Int as the focus timing T and outputs a synchronization pulse signal Sync synchronized with the focus timing T. Specifically, the laser processing apparatus 1 of this embodiment detects the focus timing T using the confocal method. Therefore, compared to detection using other focus detection methods, the focus timing T is less susceptible to the effects of surface characteristics such as inclination and roughness of the workpiece W surface, thereby improving the detection accuracy of the focus timing T.

[0079] The laser processing apparatus 1 of this embodiment further includes an imaging element 281 for imaging a workpiece W via a variable focus optical system 10. The variable focus optical system 10 includes a liquid resonance lens 12 whose refractive index periodically changes in response to an input drive signal Cf, an objective lens 11 disposed on the same optical axis as the liquid resonance lens 12, and a plurality of relay lenses 13 and 14 disposed so that the exit pupil of the objective lens 11 is conjugate with the principal point position of the liquid resonance lens 12. With this configuration, it is possible to observe an image of the workpiece W while laser processing is being performed on the workpiece W. In this embodiment, an EDOF image can be generated as an image of the workpiece W, thereby reducing the frequency of refocusing operations during image observation of the workpiece W.

[0080] Furthermore, because the variable-focus optical system 10 of this embodiment is configured so that the exit pupil of the objective lens 11 and the principal point position of the liquid resonance lens 12 are in a conjugate relationship, the magnification of the image incident on the imaging element 281 remains constant even if the focus position Pf of the variable-focus optical system 10 changes. Therefore, the field of view does not change, enabling good observation.

[0081] In addition, the advantage of using the variable focal length optical system 10 in the laser processing device 1 of this embodiment is that it can not only reduce the frequency of refocusing actions in the observation optical system 2, but also, when there is a step difference on the surface of the workpiece W, the pulsed laser Lp can be focused on the surface of the workpiece W without adjusting the position of the workpiece W.

[0082] [Variation]

[0083] The present invention is not limited to the above-described embodiment, and modifications, improvements, etc. within the scope that can achieve the object of the present invention are included in the present invention.

[0084] In the above embodiment, a pinhole may be used instead of the light guide 42. Specifically, by using a pinhole constituting a point light source and a pinhole arranged at the rear focus of the collimating lens 43, the focus timing T can be detected by the confocal method as in the above embodiment.

[0085] In the above embodiment, when fluctuations in the field of view of the observation optical system 2 are not a problem, the relay lenses 13 and 14 of the variable focal length optical system 10 can be omitted, and the collimating lens 43, the imaging lens 27, and the laser imaging lens 32 can each be combined with the objective lens 11 to form an infinity correction optical system. Alternatively, the relay lenses 13 and 14, the collimating lens 43, the imaging lens 27, and the laser imaging lens 32 can be omitted, and the objective lens 11 and the liquid resonance lens 12 can form a finite correction optical system.

[0086] In the above embodiment, the focus timing T is detected by the confocal method, but the present invention is not limited thereto. Specifically, the focus timing T may be detected by using various focus detection methods such as a double pinhole method, an astigmatism method, and a knife edge method.

[0087] For example, when using the double pinhole method, photodetectors are provided before and after a light-converging position that is conjugate with the focus position Pf, and calculations are performed based on the light detection signals output from each photodetector to determine the focus timing T. The signal processing unit 62 can output a synchronization pulse signal Sync synchronized with the focus timing T thus determined.

[0088] In the above embodiment, the illumination light Li of the observation optical system 2 is continuous light, but it can also be pulsed light. In this case, similar to the laser oscillator 311, the observation illumination light source 21 can emit pulsed light based on the synchronization pulse signal Sync. Therefore, the illumination light Li can be irradiated onto the workpiece W at the focus timing T, and the camera 28 can capture an image focused on the surface of the workpiece W.

[0089] The laser processing apparatus 1 of the above embodiment includes the observation optical system 2 for observing the workpiece W, but the present invention is not limited thereto. That is, the present invention can be applied to a laser processing apparatus that does not include the observation optical system 2 .

[0090] In the above embodiment, the signal processing unit 62 is configured in the control unit 6, but it may be configured in the lens control unit 5. In addition, the lens control unit 5 and the control unit 6 may be configured as an integrated control device.

[0091] In the above-described embodiments, the driving signal Cf is a sine wave, which causes the focus position Pf to vibrate in a sinusoidal manner. However, the driving signal Cf may be another waveform such as a triangular wave, a sawtooth wave, or a rectangular wave.

Claims

1. A laser processing device, characterized in that: have: a variable focal length optical system that periodically vibrates the focus position according to an input drive signal; a light source for irradiating detection light toward the workpiece via the variable focal length optical system; a light detector that receives the detection light reflected on the workpiece and outputs a light detection signal; a signal processing unit that, based on the input light detection signal, outputs a synchronization pulse signal synchronized with a focus timing at which the detection light is focused on the surface of the workpiece while the light detection signal is equal to or greater than a threshold value; a laser oscillator that oscillates a pulsed laser based on the input synchronization pulse signal and in accordance with the focus timing, and irradiates the workpiece with the pulsed laser via the variable focal length optical system; The output period of the pulse laser light once is set to be shorter than the change cycle of the focus position.

2. The laser processing device according to claim 1, wherein A laser processing mask is further provided. The laser processing mask is arranged between the variable focus optical system and the laser oscillator, is located at a position conjugate to the focus position of the variable focus optical system, and has an opening through which the pulsed laser light passes.

3. The laser processing device according to claim 1, wherein The light detector is configured so that the light detection signal reaches a peak value when the focus position of the variable focal length optical system coincides with the surface of the workpiece. The signal processing unit detects the peak value of the light detection signal as the focus timing, and outputs the synchronization pulse signal synchronized with the focus timing.

4. The laser processing device according to any one of claims 1 to 3, wherein: It also includes a photographing element for photographing the workpiece through the variable focal length optical system. The variable focal length optical system has: a liquid resonant lens, the refractive index of which changes periodically according to the input drive signal; an objective lens disposed on the same optical axis as the liquid resonant lens; A plurality of relay lenses are arranged so that the exit pupil of the objective lens is conjugate with the principal point position of the liquid resonance lens.

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