Light spot detection method, focusing method and photoacoustic measurement system

Abstract

The application provides a light spot detection method, a focusing method and a photoacoustic measurement system. The light spot detection method comprises: adjusting a relative optical deflection angle between excitation light and detection light through a light beam deflection module, collecting reflected light of the detection light on a sample surface in real time through a detection module, and generating a signal strength indicating the reflected light; fitting a relationship curve of the signal strength with respect to the relative optical deflection angle; obtaining a first target relative optical deflection angle θ1 and a second target relative optical deflection angle θ2 based on the relationship curve; and calculating an effective light spot size, which is the sum of the size of the excitation light spot and the size of the detection light spot in the direction of relative movement. The method adjusts the relative optical deflection angle between the excitation light and the detection light, fits the relationship curve of the signal strength with respect to the relative optical deflection angle, and then measures the effective light spot size. The photoacoustic measurement system is focused based on the light spot detection method, and further realizes effective and accurate film thickness measurement.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, specifically to a spot detection method, a focusing method, and a photoacoustic measurement system. Background Technology

[0002] Photoacoustic measurement systems are used to measure the thickness of various films, such as metal films and dielectric films, using the photoacoustic effect. The principle of the photoacoustic effect is as follows:

[0003] like Figure 1 and 2 As shown, when excitation light is irradiated onto the sample surface, due to the photoacoustic effect, the sample absorbs photons and accumulates energy, thereby generating thermal stress. This thermal stress induces thermoelastic deformation. Figure 2 The raised position S shown in the figure generates a photoacoustic signal.

[0004] like Figure 3 As shown, the photoacoustic signal propagates on the sample surface and inside. The longitudinal sound wave propagates to the interface, generating the first echo signal h1. This first echo signal h1 reaches the upper surface, causing a change in the material's reflectivity. Simultaneously, the first echo signal h1 bounces off the upper surface and then bounces back to the interface, generating a second echo signal h2. This second echo signal h2 reaches the upper surface, causing another change in the material's reflectivity.

[0005] The principle of the photoacoustic measurement system for measuring film thickness is as follows:

[0006] like Figure 4 As shown, sample 41 is illuminated with excitation light, and probe light is directed onto the excitation light spot. Since the reflectivity changes during echo transmission, the signal intensity of the reflected light is collected by the detection module 42. Combined with... Figure 5 As shown, the detection module obtains the change in detection light power caused by the excitation light, thereby obtaining the time interval Δt between the two echoes, and obtaining the film thickness value through the film thickness calculation formula.

[0007] Improving the accuracy of film thickness measurement is a major challenge in the field of photoacoustic measurement. Summary of the Invention

[0008] In view of the problems in the prior art, the purpose of this invention is to provide a spot detection method, a focusing method, and a photoacoustic measurement system to solve the problem of film thickness measurement accuracy in related technologies.

[0009] This invention provides a method for detecting light spots, comprising:

[0010] Excitation light and probe light are generated by the light source module. The excitation light and probe light are respectively incident on the sample surface within the focal depth range of the focusing module along their respective optical paths through the focusing module, forming excitation light spot and probe light spot on the sample surface.

[0011] The incident direction of the excitation light or the incident direction of the probe light is adjusted by the beam deflection module, so that the excitation light spot and the probe light spot move relative to each other on the sample surface. The relative optical deflection angle between the excitation light and the probe light is obtained in real time. The probe module collects the reflected light of the probe light on the sample surface in real time and generates a signal intensity indicating the reflected light. This adjustment makes the signal intensity go through a process of increasing from the background noise region value R0 and then decreasing back to the background noise region value R0.

[0012] Fit the curve showing the relationship between signal intensity and relative optical deflection angle;

[0013] The relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target are obtained based on the relationship curve;

[0014] Based on the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target, the effective spot size is calculated. The effective spot size is the sum of the size of the excitation spot and the size of the probe spot in the direction of relative movement.

[0015] The spot detection method provided by this invention can provide a spot detection method without using a camera or the traditional blade edge method. By adjusting the relative optical deflection angle between the excitation light and the probe light and fitting the relationship curve of the signal intensity with respect to the relative optical deflection angle, the effective spot size can be obtained, that is, the sum of the excitation light spot size and the probe light size, without the need to obtain the excitation light spot size and the probe light size separately, thus reducing the cost of measuring the spot size. Furthermore, due to the high signal intensity sensitivity in this method, the accuracy of the obtained effective spot size is high.

[0016] Compared to related technologies that use cameras to measure spot size, the spot detection method of this invention overcomes the limitations of camera resolution, thereby improving the accuracy of the effective spot size. Furthermore, compared to related technologies that use the blade boundary method to measure spot size, the spot detection method of this invention is more efficient and eliminates the need for preparing high-precision pattern samples and equipping high-precision X / Y / Z / Rz translation stages, thus reducing the cost of measuring spot size.

[0017] This disclosure also provides a focusing method applied to a photoacoustic measurement system, including:

[0018] The focal depth of the focusing module of the photoacoustic measurement system is obtained. Within the focal depth range, the position of the sample is adjusted along a direction perpendicular to the sample surface and / or the position of the focusing module is adjusted along the optical axis of the focusing module to change the sample spacing, wherein the sample spacing is the distance between the focusing module and the sample.

[0019] According to the spot detection method of any of the above embodiments, the effective spot size corresponding to each sample spacing is obtained respectively;

[0020] Obtain the minimum value among the multiple effective light spot sizes, adjust the current sample spacing to the sample spacing corresponding to the minimum value, and complete the focusing of the photoacoustic measurement system.

[0021] Traditional spot size measurement methods yield essentially unchanged spot sizes within the depth of focus, failing to pinpoint the precise location of the smallest spot. However, the focusing method provided by this invention can determine the effective spot size at different locations within the depth of focus, thereby enabling the precise determination of the focal plane position corresponding to the smallest spot within that range.

[0022] This disclosure also provides another focusing method, applied to a photoacoustic measurement system, including:

[0023] The focal depth of the focusing module of the photoacoustic measurement system is obtained. Within the focal depth range, the position of the sample is adjusted along a direction perpendicular to the sample surface based on a first adjustment spacing and / or the position of the focusing module is adjusted along the optical axis of the focusing module based on the first adjustment spacing to change the sample spacing. The sample spacing is the distance between the focusing module and the sample. According to the spot detection method of any of the above embodiments, the effective spot size corresponding to each sample spacing is obtained to obtain a first set of effective spot sizes.

[0024] Obtain the minimum value among the first group of effective spot sizes, and adjust the current sample spacing to the sample spacing corresponding to the minimum value;

[0025] The sample spacing is adjusted based on a second adjustment distance along the direction perpendicular to the sample surface and / or along the optical axis of the focusing module, and the relationship curve corresponding to each sample spacing is obtained according to the spot detection method described in any of the above embodiments; wherein, the second adjustment distance is smaller than the first adjustment distance;

[0026] The maximum value corresponding to each of the relationship curves is obtained, resulting in multiple maximum values. The maximum value among the multiple maximum values ​​is determined, and the current sample spacing is adjusted to the sample spacing corresponding to the maximum value, thus completing the focusing of the photoacoustic measurement system.

[0027] The focusing method provided by this invention first determines the sample spacing corresponding to the smallest effective spot based on the effective spot size corresponding to different positions within the depth of focus range. On this basis, the sample spacing is adjusted with a smaller adjustment interval, and the precise position corresponding to the smallest spot is further confirmed by the change of the highest point of high-sensitivity signal intensity, thus obtaining a high-precision focal plane position and further improving the focusing accuracy.

[0028] This disclosure also provides a photoacoustic measurement system, which includes:

[0029] The light source module is used to generate excitation and probe light;

[0030] A beam combiner is used to receive excitation and probe light and to combine the excitation and probe light together.

[0031] A beam deflection module is disposed in the excitation light optical path and / or the probe light optical path to adjust the relative optical deflection angle between the excitation light and the probe light.

[0032] A focusing module is used to converge the beams after they have been combined by the beam combiner to form an excitation light spot and a probe light spot on the sample surface, and to collect the reflected light of the probe light on the sample surface.

[0033] The detection module is used to collect the reflected light of the detection light on the sample surface and generate a signal intensity indicating the reflected light.

[0034] The control module is used to complete the focusing of the photoacoustic measurement system according to the focusing method of any of the above embodiments;

[0035] The data processing module outputs the parameters to be measured based on the signal intensity of the reflected light after the focusing is completed.

[0036] The photoacoustic measurement system provided by this invention adjusts the relative optical deflection angle of the excitation and probe light by setting a beam deflection module, thereby achieving the focusing method provided in any embodiment. The resolution for the positional offset of the light spot can reach the 10nm level, thus obtaining an effective light spot size with an accuracy at the 10nm level. Furthermore, within the depth of focus, the precise position corresponding to the smallest light spot is further confirmed by the change in the point of highest signal intensity with high sensitivity, ensuring measurement at a high-precision focal plane position, significantly improving signal strength, and thus improving measurement accuracy. Attached Figure Description

[0037] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.

[0038] Figures 1-3 This is a schematic diagram illustrating the principle of photoacoustic effect;

[0039] Figures 4-5 A schematic diagram illustrating the film thickness testing principle based on a photoacoustic measurement system in related technologies is shown.

[0040] Figure 6 A flowchart illustrating the spot detection method provided in this embodiment is shown.

[0041] Figure 7 Demonstration and use Figure 6 Discrete data of the relative optical deflection angle and its corresponding signal intensity acquired by the spot detection method shown;

[0042] Figure 8 Display based on Figure 7 The curve shown represents the relationship between the relative optical deflection angle and the corresponding signal intensity obtained by fitting discrete data.

[0043] Figure 9 Demonstration and use Figure 6 A schematic diagram illustrating the relative positional changes of the excitation light spot and the probe light spot during the light spot detection process shown;

[0044] Figure 10 Showing Figure 6 The diagram illustrates the principle of adjusting the relative optical deflection angle in the spot detection method shown.

[0045] Figure 11 A flowchart illustrating a spot detection method provided in another embodiment of this disclosure is shown.

[0046] Figure 12 The curve showing the relationship between the position D of the optical delay line and the signal intensity R of the reflected light from the probe light is displayed.

[0047] Figure 13 A flowchart illustrating the focusing method provided in this disclosure embodiment is shown;

[0048] Figure 14 This presents a flowchart illustrating another embodiment of the method for focusing another object.

[0049] Figure 15 A schematic diagram of a Gaussian beam is shown;

[0050] Figure 16 One of the schematic diagrams showing the structure of the photoacoustic measurement system according to an embodiment of the present disclosure;

[0051] Figure 17 A second schematic diagram showing the structure of the photoacoustic measurement system according to an embodiment of the present disclosure;

[0052] Figure 18 The third schematic diagram showing the structure of the photoacoustic measurement system according to an embodiment of the present disclosure;

[0053] Figure 19The fourth schematic diagram shows the structure of the photoacoustic measurement system according to an embodiment of the present disclosure. Detailed Implementation

[0054] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided so that the invention will be more comprehensive and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0055] Furthermore, the accompanying drawings are merely illustrative of the invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0056] The flowchart shown in the attached diagram is merely an illustrative example and does not necessarily include all steps. For example, some steps may be broken down, while others may be combined or partially combined. Therefore, the actual execution order may change depending on the specific circumstances.

[0057] In photoacoustic measurement, the focused spot size of the probe and excitation beams affects the signal intensity acquired by the detection module, thus impacting the accuracy of film thickness measurement. Furthermore, research on the photoacoustic measurement system revealed that variations in the light source, vibration, optomechanical stress, changes in photoelectric control devices, and material temperature drift can cause the probe and / or excitation beam spots to defocus, resulting in changes in the focused spot size on the sample surface. This leads to a deterioration of the photoacoustic signal in the measurement system, resulting in low film thickness measurement accuracy.

[0058] Therefore, monitoring the probe and excitation light spots of the photoacoustic measurement system by detecting the effective spot size is a factor in improving the accuracy of film thickness measurement.

[0059] Currently, there are two methods for detecting the size of the light spot on the sample surface:

[0060] One method involves observing the size of the light spot scattered from the sample surface using an industrial camera perpendicular to the sample surface. Another method uses the traditional knife-edge method, which generally involves moving a high-precision pattern sample using a high-precision X / Y / Z / Rz translation stage, causing the knife-edge of the high-precision pattern sample to move relative to the light spot. The size of the light spot is then determined based on the light signal reflected from this boundary.

[0061] The two methods for measuring spot size require measuring the excitation spot size and the probe spot size separately. The first method is limited by the camera resolution and has low accuracy in obtaining the spot size, while the second method is costly, inefficient, and time-consuming.

[0062] Figure 6 A flowchart illustrating the spot detection method provided in the embodiments of this disclosure is shown, such as... Figure 6 As shown, the spot detection method includes, but is not limited to, the following steps:

[0063] Step 610: Excitation light and probe light are generated by the light source module. The excitation light and probe light are respectively incident on the sample surface within the focal depth range of the focusing module along their respective optical paths through the focusing module, forming excitation light spot and probe light spot on the sample surface.

[0064] Step 620: Adjust the incident direction of the excitation light or the incident direction of the probe light through the beam deflection module, so that the excitation light spot and the probe light spot move relative to each other on the sample surface. The relative optical deflection angle between the excitation light and the probe light is acquired in real time. The probe module collects the reflected light of the probe light on the sample surface in real time and generates a signal intensity indicating the reflected light. This adjustment makes the signal intensity go through a process of increasing from the background noise region value R0 and then decreasing back to the background noise region value R0.

[0065] Step 630: Fit the curve showing the relationship between signal intensity and relative optical deflection angle;

[0066] Step 640: Obtain the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target based on the relationship curve;

[0067] Step 650: Based on the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target, calculate the effective spot size. The effective spot size is the sum of the size of the excitation light spot and the size of the probe light spot in the direction of relative movement.

[0068] This disclosure provides a spot detection method without using a camera or traditional blade edge method. By adjusting the relative optical deflection angle between the excitation light and the probe light and fitting the relationship curve of signal intensity with respect to the relative optical deflection angle, the effective spot size can be obtained, which is the sum of the excitation light spot size and the probe light size, without the need to obtain the excitation light spot size and the probe light size separately. This reduces the cost of measuring spot size. Furthermore, due to the high signal intensity sensitivity in this method, the accuracy of the obtained effective spot size is high.

[0069] Compared to related technologies that use cameras to measure spot size, the spot detection method of this invention overcomes the limitations of camera resolution, thereby improving the accuracy of the effective spot size. Furthermore, compared to related technologies that use the blade boundary method to measure spot size, the spot detection method of this invention is more efficient and eliminates the need for preparing high-precision pattern samples and equipping high-precision X / Y / Z / Rz translation stages, thus reducing the cost of measuring spot size.

[0070] In this embodiment, the incident direction of the excitation light or the incident direction of the probe light is adjusted by a beam deflection module, causing the excitation light spot and the probe light spot to move relative to each other on the sample surface. The relative optical deflection angle θ of the probe light beam relative to the excitation light beam is acquired in real time. Simultaneously, the probe module collects the reflected light from the sample surface in real time and generates a signal intensity R(θ) indicating the reflected light. During the adjustment process, the signal intensity undergoes a process of increasing from the background noise region value R0 and then decreasing back to the background noise region value R0. Figure 7 As shown, when the relative optical deflection angle gradually increases, the signal intensity first gradually increases and then gradually decreases. The discrete dots represent multiple sampling points of the signal intensity R(θ) of the relative optical deflection angle θ and the corresponding probe light reflected light.

[0071] Figure 7 In the multiple discrete dot data shown, the regions where the signal strength is no greater than the background noise value of the detection module include the background noise region on the left and the background noise region on the right. The average signal strength of at least one of the two background noise regions is taken, and this average value is represented as the background noise region value R0. In addition, in one embodiment, the background noise value of the detection module can be directly used as the background noise region value R0.

[0072] In this embodiment of the disclosure, based on the light intensity distribution function of the light spot on the sample surface, the light intensity distribution function is... Figure 7By fitting the data from multiple discrete dots shown, the relationship curve between signal intensity and relative optical deflection angle can be obtained. For example, both the excitation and probe beams are Gaussian beams, meaning the intensity distribution of the excitation and probe beams on the sample surface is Gaussian. Therefore, the signal intensity relative to the relative optical deflection angle θ follows a Gaussian distribution, and the curve of signal intensity R(θ) with respect to the relative optical deflection angle θ is approximately a Gaussian function curve. Therefore, a Gaussian function is used for fitting, and the formula for the Gaussian function is shown below:

[0073] Where a, b and c are real constants obtained from the fitting, and a>0, θ represents the relative optical deflection angle, and R(θ) represents the signal intensity.

[0074] Based on this, the fitting yields Figure 8 The solid line shown represents the relationship between signal intensity and the relative optical deflection angle. Furthermore, the change in signal intensity reflects the change in the intensity of the reflected light, and this change is due to the change in the positional relationship between the probe light spot and the excitation light spot—that is, the change in the overlap state between the measurement light spot and the excitation light spot—during the adjustment of the beam deflection module. Therefore… Figure 8 The resulting relationship curve shows that the change in the relative optical deflection angle corresponds to the change in the relative position between the probe light spot and the excitation light spot; that is, different relative optical deflection angles correspond to different overlap states of the two spots on the sample surface. Therefore, using... Figure 8 The change in signal intensity in the relationship curve shown can be used to obtain the relative optical deflection angle corresponding to different overlap states between the probe light spot and the excitation light spot. The effective spot size can be obtained based on the relative movement distance between the probe light spot and the excitation light spot during the process of changing the overlap state between the probe light spot and the excitation light spot. Therefore, the effective spot size can be calculated based on the relationship curve.

[0075] In the embodiments disclosed herein, such as Figure 8 As shown, the regions on both sides of the relationship curve are background noise regions. For example, a signal strength threshold for the background noise region is set as the background noise cutoff value Rmin. The relative optical deflection angle corresponding to the background noise cutoff value Rmin is obtained from the relationship curve and used as the target relative optical deflection angle. Further, obtaining the first target relative optical deflection angle θ1 and the second target relative optical deflection angle θ2 based on the relationship curve includes:

[0076] Obtain the peak signal intensity Rmax of the relationship curve;

[0077] Obtain the background noise cutoff value Rmin from the relationship curve;

[0078] In the relationship curve, the relative optical deflection angle corresponding to the decrease in signal strength from the first side of the peak signal strength Rmax to the background noise cutoff value Rmin is taken as the first target relative optical deflection angle θ1, and the relative optical deflection angle corresponding to the decrease in signal strength from the second side of the peak signal strength Rmax to the background noise cutoff value Rmin is taken as the second target relative optical deflection angle θ2. As shown in the figure, the abscissa values ​​of the two points where R(θ)=Rmin intersects with the relationship curve correspond to the first target relative optical deflection angle θ1 and the second target relative optical deflection angle θ2, respectively.

[0079] like Figure 8 The relationship curves shown indicate that when the relative optical deflection angle θ ≤ θ1 or θ ≥ θ2, the regions on both sides of the curve represent background noise, and the corresponding signal intensity is not greater than the background noise cutoff value. This indicates that the intensity of the light reflected from the probe light on the sample surface is not affected by the excitation light, and the detection module does not detect a photoacoustic signal, meaning that the probe light spot and the excitation light spot are in a non-overlapping, separated state. When the relative optical deflection angle θ1 < θ < θ2, the corresponding signal intensity is greater than the background noise cutoff value, indicating that the intensity of the light reflected from the probe light on the sample surface is affected by the excitation light, and the detection module detects a photoacoustic signal. At this time, the probe light spot and the excitation light spot on the sample surface at least partially overlap and are in an overlapping state. When the relative optical deflection angle θ = θb, the corresponding signal intensity value is the peak signal intensity Rmax of the relationship curve, indicating that the intensity of the light reflected from the probe light on the sample surface is most strongly affected by the excitation light, and the detection module detects the strongest photoacoustic signal. At this time, the center of the probe light spot and the center of the excitation light spot on the sample surface completely overlap.

[0080] The following combination Figure 9 The display and Figure 8 The schematic diagram showing the relative position changes of the excitation beam spot and the probe beam spot corresponding to the relationship curve further illustrates the above embodiment. By adjusting the beam deflection module to move the probe beam spot 72, the relative optical deflection angle θ between the probe beam and the excitation beam is changed, corresponding to the positional relationship between the probe beam spot 72 and the excitation beam spot 71. When the relative optical deflection angle is equal to θ1, it corresponds to... Figure 9 At position A, the probe light spot 72 and the excitation light spot 71 are exactly offset / separated; when the relative optical deflection angle is equal to the abscissa value θb corresponding to the peak signal intensity Rmax of the relationship curve, it corresponds to... Figure 9 At position B, the center of the probe light spot 72 completely coincides with the center of the excitation light spot 71; when the relative optical deflection angle is equal to θ2, it corresponds to... Figure 9At position C, the probe light spot 72 and the excitation light spot 71 are once again completely offset / separated. During this process, when the probe light spot 72 moves from position A to position C, the relative movement distance ΔD between the probe light spot 72 and the excitation light spot 71 is the sum of the size of the probe light spot 72 and the size of the excitation light spot 71, that is, the relative movement distance ΔD is equal to the effective spot size.

[0081] Furthermore, the principle of obtaining the effective spot size based on adjusting the relative optical deflection angle in the above-mentioned spot detection method is explained, in conjunction with... Figure 10 As shown, the probe beam 7a and the excitation beam 7b are focused onto the sample surface by the focusing module 7, respectively, and the beam deflection module is used to... Figure 10 The incident direction of the probe beam 7a or the excitation beam 7b is adjusted to cause a relative movement Δd between the excitation beam spot and the probe beam spot on the sample surface. This relative movement Δd characterizes the change in the relative position between the centers of the excitation beam spot and the probe beam spot. Taking the focusing module 7 using a lens as an example, and the distance between the lens and the sample surface being the focal length f, when the probe beam 7a undergoes an optical deflection angle θ relative to the excitation beam 7b, since the centerlines of both beams pass through the center of the lens, geometric relationships indicate that:

[0082] Therefore, it can be deduced that, combined with Figure 9 As shown, when the probe spot 72 moves from position A to position C, the relative movement distance ΔD between the probe spot 72 and the excitation spot 71 satisfies the following relationship:

[0083] ΔD=|f*tan(θ1)-f*tan(θ1)|≈|θ1-θ2|*f.

[0084] Therefore, in one embodiment, the effective spot size is calculated based on the optical deflection angle θ1 of the first target and the optical deflection angle θ2 of the second target, including calculating the effective spot size using the following formula: D=|θ1-θ2|*f, where D is the effective spot size and f is the focal length of the focusing module.

[0085] As can be seen from the above expression, by selecting a suitable beam deflection module and focusing module 7, the resolution Δd of the relative movement between the excitation light spot and the probe light spot on the sample surface can be made much smaller than the spot size. For example, Δd can be made less than 1 / 10 of the spot size. In one embodiment, the beam deflection module uses a piezoelectric deflection mirror, which can achieve a resolution of 10 nm between the two spots, thus obtaining an effective spot size accuracy of 10 nm. Therefore, by selecting a suitable beam deflection module and focusing module, the resolution of the relative movement between the excitation light spot and the probe light spot can be ensured to be sufficiently small, allowing the spot detection method provided in this embodiment to obtain an effective spot size with sufficiently high accuracy. Compared to existing techniques that use cameras to measure spot size, the method provided in this embodiment significantly overcomes the limitations of camera resolution; furthermore, this embodiment does not require the preparation of high-precision pattern samples or the use of high-precision X / Y / Z / Rz translation stages, improving the accuracy of spot size acquisition while also reducing the cost of measuring spot size.

[0086] In embodiments of this disclosure, the background noise cutoff value Rmin satisfies the following relationship:

[0087] Rmin = R0.

[0088] When the background noise cutoff value Rmin is the same as the background noise region value R0, the first relative optical deflection angle θ1 and the second relative optical deflection angle θ2 corresponding to the background noise cutoff value Rmin can be directly obtained from the relationship curve.

[0089] In another embodiment of this disclosure, the background noise cutoff value Rmin satisfies the following relationship:

[0090] Rmin=(Rmax-R0)*k*(1 / e 2 ), k∈[1, e 2 ).

[0091] When the signal strength is k*(1 / e) of the difference between the peak signal strength Rmax and the background noise region value R0 2 When k∈[1, e) 2 This is the boundary contact position between the probe light spot and the excitation light spot. For example, taking k=1, the difference between Rmax and R0 (1 / e) 2 At point ), the slope of the relationship curve is larger, and it is less affected by noise. The signal strength is more sensitive at this point, and the signal-to-noise ratio is high. Therefore, the boundary contact position between the probe spot and the excitation spot is more accurate, and the final calculated effective spot size is more accurate.

[0092] In one embodiment of this disclosure, such as Figure 9As shown, the direction of relative movement is the direction of the major axis of the excitation light spot 71. In this case, the effective spot size is equal to the sum of the major axes of the detector light spot 72 and the excitation light spot 71, wherein the direction of the major axis of the excitation light spot is the same as the direction of the major axis of the detector light spot. Alternatively, not shown in the figure, the direction of relative movement can also be the direction of the minor axis of the excitation light spot. In this case, the effective spot size is the sum of the minor axes of the excitation light spot and the detector light spot, wherein the direction of the minor axis of the excitation light spot is the same as the direction of the minor axis of the detector light spot. Figure 9 The light spot shown is elliptical in shape. This is just an example; the shape of the light spot is not limited to ellipse and can also be other shapes.

[0093] The methods for obtaining the effective spot size in the embodiments of this disclosure do not rely on a camera, avoiding the problem of spot size acquisition by optical images being limited by camera resolution, thus improving the measurement accuracy of the effective spot size. In addition, compared with the related technology that uses the blade boundary method to measure the spot size, the spot detection methods in the embodiments of this disclosure do not require the preparation of high-precision pattern samples and the equipment of high-precision X / Y / Z / Rz translation stages, which can reduce the cost of measuring the spot size and obtain the effective spot size with high efficiency and high accuracy; on this basis, the film thickness measurement accuracy can be further improved.

[0094] like Figure 11 As shown in the figure, this disclosure also provides a flowchart of another spot detection method, which includes, but is not limited to, the following steps:

[0095] Step 1210: Excitation light and probe light are generated by the light source module. The excitation light and probe light are respectively incident on the sample surface within the focal depth range of the focusing module along their respective optical paths through the focusing module, forming excitation light spot and probe light spot on the sample surface.

[0096] Step 1220: Adjust the optical path of the excitation light and / or the optical path of the probe light so that the optical path of the excitation light and the optical path of the probe light are equal;

[0097] Step 1230: Adjust the incident direction of the excitation light or the incident direction of the probe light through the beam deflection module, so that the excitation light spot and the probe light spot move relative to each other on the sample surface. The relative optical deflection angle between the excitation light and the probe light is acquired in real time. The probe module collects the reflected light of the probe light on the sample surface in real time and generates a signal intensity indicating the reflected light. This adjustment makes the signal intensity go through a process of increasing from the background noise region value R0 and then decreasing back to the background noise region value R0.

[0098] Step 1240: Fit the curve showing the relationship between signal intensity and relative optical deflection angle;

[0099] Step 1250: Obtain the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target based on the relationship curve;

[0100] Step 1260: Based on the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target, calculate the effective spot size. The effective spot size is the sum of the size of the excitation light spot and the size of the probe light spot in the direction of relative movement.

[0101] Combination Figure 12 As shown, taking the adjustment of the optical delay line position as an example, Figure 12 The figure shows the relationship between the optical delay line position D and the signal intensity R of the reflected light from the probe. This figure is a typical schematic diagram of the signal in photoacoustic measurement of a metal film. The signal intensity reaches its maximum value R at this point. M The position D0 of the optical delay line corresponding to the time is denoted as the zero point position, and correspondingly, R M This is the zero-point signal output by the detection module. The zero-point position is also the position of the optical delay line corresponding to the equal optical path lengths of the excitation and detection beams.

[0102] Therefore, by adjusting the position of the optical delay line to obtain the maximum signal strength, a high signal-to-noise ratio (SNR) signal strength can be obtained when subsequently adjusting the relative optical deflection angle. A high SNR further improves the measurement accuracy of the effective spot size, thereby enhancing the monitoring precision of the photoacoustic measurement system. Therefore, before performing step 1230, the optical delay line is fixed at the zero-point position D0. Alternatively, the optical delay line can be fixed at other positions, such as near the zero-point position, as long as the signal strength and SNR of the signal output by the detection module that collects the reflected light from the probe light meet the requirements.

[0103] The spot detection method provided in the embodiments of this disclosure can be applied to film thickness measurement schemes based on photoacoustic measurement systems. It can be used to obtain the effective spot size of the photoacoustic measurement system. Since this method can achieve accurate spot detection, calibrating the photoacoustic measurement system based on the effective spot size can further achieve effective and accurate film thickness measurement. In addition, the effective spot size obtained in the above embodiments is the sum of the probe spot size and the excitation spot size in the direction of relative movement. If the probe spot size and / or the excitation spot size changes, the effective spot size also changes accordingly. Therefore, the spot detection method provided in this disclosure can replace the traditional method of measuring spot size (obtaining the excitation spot size and probe spot size separately), and can be used for stability analysis of photoacoustic measurement systems and intensity analysis of excitation photoacoustic signals in photoacoustic measurement systems.

[0104] This disclosure also provides a focusing method applied to a photoacoustic measurement system. For example... Figure 13As shown, the focusing method in this embodiment may specifically include the following steps:

[0105] Step 1410: Obtain the depth of focus of the focusing module of the photoacoustic measurement system. Within the depth of focus, adjust the position of the sample along the direction perpendicular to the sample surface and / or adjust the position of the focusing module along the optical axis of the focusing module to change the sample spacing. The sample spacing is the distance between the focusing module and the sample.

[0106] According to the spot detection method provided in the embodiments of this disclosure, the effective spot size corresponding to each sample spacing is obtained respectively;

[0107] Step 1420: Obtain the minimum value among multiple effective spot sizes, adjust the current sample spacing to the sample spacing corresponding to the minimum value, and complete the focusing of the photoacoustic measurement system.

[0108] Specifically, the excitation light and probe light generated by the photoacoustic measurement system are focused by the focusing module and then irradiate the sample surface. After applying the above focusing method, the sample surface becomes the focal plane of the focusing module, thus completing the focusing of the photoacoustic measurement system.

[0109] The principle of this embodiment is as follows: In the photoacoustic measurement system, the excitation beam and the probe beam are focused onto the sample surface by a focusing module. When the distance between the sample and the focusing module changes, the sizes of the excitation beam spot and the probe beam spot on the sample surface increase or decrease together. When both the excitation beam spot and the probe beam spot are at their smallest sizes, the effective spot size is also the smallest. At this point, the photoacoustic signal excited by the photoacoustic measurement system is the strongest, and the position of the sample surface at this time is the position of the focal plane. This focal plane position is maintained for subsequent film thickness measurement.

[0110] In related technologies, methods for focusing or determining the focal plane generally involve adjusting the position of the condenser lens along the optical axis or controlling the motion stage to adjust the sample's position along the Z-axis. At different positions, conventional spot size measurement methods are used to detect the spot size and the corresponding focal plane position. This method has low resolution in determining the focal plane position; within the condenser lens's depth of focus, the acquired spot size remains essentially unchanged, and it cannot further determine the precise position corresponding to the smallest spot. Compared to related technologies, this embodiment can determine the effective spot size corresponding to different positions within the depth of focus, thereby enabling the further determination of the precise focal plane position corresponding to the smallest spot within the depth of focus.

[0111] This disclosure also provides another focusing method, which is applied to a photoacoustic measurement system. For example... Figure 14 As shown, the focusing method in this embodiment may specifically include the following steps:

[0112] Step 1510: Obtain the focal depth of the focusing module of the photoacoustic measurement system. Within the focal depth range, adjust the position of the sample along the direction perpendicular to the sample surface based on the first adjustment spacing and / or adjust the position of the focusing module along the optical axis direction of the focusing module based on the first adjustment spacing to change the sample spacing. The sample spacing is the distance between the focusing module and the sample. According to the spot detection method provided in the embodiments of this disclosure, obtain the effective spot size corresponding to each sample spacing to obtain the first set of effective spot sizes.

[0113] Step 1520: Obtain the minimum value among the effective spot sizes in the first group, and adjust the current sample spacing to the sample spacing corresponding to the minimum value;

[0114] Step 1530: Adjust the sample spacing based on the second adjustment spacing along the direction perpendicular to the sample surface and / or along the optical axis of the focusing module, and obtain the relationship curve corresponding to each sample spacing according to the spot detection method provided in this embodiment of the present disclosure; wherein, the second adjustment spacing is smaller than the first adjustment spacing;

[0115] Step 1540: Obtain the maximum value corresponding to each of the relationship curves, obtain multiple maximum values, determine the maximum value among the multiple maximum values, and adjust the current sample spacing to the sample spacing corresponding to the maximum value to complete the focusing of the photoacoustic measurement system.

[0116] For example, at different Z-axis positions, the corresponding effective spot size is obtained, and the Z-axis position corresponding to the smallest effective spot size is set as the coarse focal plane. For example, in this method, the first adjustment distance for obtaining different Z-axis positions can be on the order of μm, such as 10 μm or other values. For example, after executing step 1520, the corresponding sample surface is the coarse focal plane. Compared with the prior art, the coarse focal plane obtained at this time further determines the position corresponding to the smallest spot within the depth of focus, improving focusing accuracy.

[0117] Based on the established coarse focal plane, a high-precision focal plane can be further obtained. At this point, using the coarse focal plane as a reference, different Z-axis positions can be adjusted with a smaller second adjustment interval; for example, the second adjustment interval could be 100 nm. When the effective spot size is nearly identical at different Z-axis positions or Z-axis height positions (e.g., within ±1 μm of the coarse focal plane), the maximum signal intensity is obtained under different Z-axis position conditions. That is, using the spot detection method provided in this embodiment, the corresponding relationship curves are obtained under different Z-axis position bars, and the maximum value of each relationship curve is obtained to obtain the maximum values ​​corresponding to different Z-axis positions. The maximum signal intensity under different Z-axis position conditions is then compared, and the Z-axis position corresponding to the largest signal intensity (i.e., the largest among multiple maximum values) is the high-precision focal plane, further improving focusing accuracy.

[0118] The technical effect of this embodiment is explained as follows: the axial resolution Δdof (depth of focus) of the focusing lens can be expressed by the following formula:

[0119]

[0120] For a photoacoustic measurement system, assuming the incident beam diameter is 2 mm, λ = 515 nm, and NA = 0.02, substituting these values ​​into the formula yields Δdof ≈ 1.3 mm. Within a 1.3 mm region near the focal point of the Gaussian beam, such as... Figure 15 As shown, the Gaussian beam has almost uniform spot sizes near the beam waist (e.g., region s). The traditional knife-edge method, based on measuring the light intensity of the spot, has low signal sensitivity and is limited by factors such as the responsivity and signal-to-noise ratio of the detection module. Therefore, this traditional method cannot accurately determine the location of the smallest spot within the depth of focus. Furthermore, using a camera to acquire the spot size also yields almost uniform spot sizes within this depth of focus, failing to determine the location of the smallest spot within that range.

[0121] In photoacoustic effects, the acoustic waves generated after the sample surface absorbs excitation light radiation cause changes in the reflectivity of the probe light primarily due to electron heating. Furthermore, the photoacoustic effect is highly sensitive to changes in optical power density; different spot sizes at different Z-axis height positions lead to variations in optical power density, further resulting in changes in the point of highest signal intensity. The detection sensitivity of signal intensity is far higher than the monitoring resolution at Z-axis height positions. Monitoring the focusing accuracy by measuring the reflected signal intensity of the probe light can reach the nanometer level, thus serving as a direct monitoring item for the final system measurement performance and exhibiting high correlation with system performance. Therefore, using this embodiment, even within the depth of focus range, the precise location corresponding to the smallest spot, i.e., the high-precision focal plane position, can be further confirmed by observing changes in the point of highest signal intensity with high sensitivity.

[0122] This disclosure also provides a photoacoustic measurement system for implementing the focusing method of the above embodiments, such as... Figure 16 As shown, the photoacoustic measurement system may include:

[0123] Light source module 171 is used to generate laser light;

[0124] Beam splitter 177 is used to split the laser emitted by light source module 171 into excitation light and probe light;

[0125] A beam combiner 172 is used to receive excitation light and probe light, and to combine the excitation light and probe light.

[0126] Beam deflection module 173 is disposed in the excitation light optical path and / or the probe light optical path to adjust the relative optical deflection angle between the excitation light and the probe light;

[0127] The focusing module 174 is used to converge the beam after it has been combined by the beam combiner 172 to form an excitation light spot and a probe light spot on the sample surface, and to collect the reflected light of the probe light on the sample surface.

[0128] The detection module 175 is used to collect the reflected light of the detection light on the sample surface and generate a signal intensity indicating the reflected light.

[0129] Control module 176 is used to complete the focusing of the photoacoustic measurement system according to the focusing method provided in the embodiments of this disclosure;

[0130] The data processing module 177 outputs the parameters to be measured based on the signal intensity of the reflected light after the focusing is completed.

[0131] In this embodiment, the relative optical deflection angle between the excitation light and the probe light is adjusted by setting a beam deflection module 173 in the photoacoustic measurement system, thereby achieving... Figure 13 or Figure 14 The focusing method described achieves a resolution of 10nm for the positional offset of the light spot, thus obtaining an effective light spot size with an accuracy of 10nm. Furthermore, within the depth of focus, the precise position corresponding to the smallest light spot is further confirmed by observing changes in the point of highest signal intensity with high sensitivity, thereby ensuring measurement at a high-precision focal plane position and significantly improving signal strength, which in turn improves measurement accuracy. Simultaneously, this embodiment does not require high-precision patterned samples (conventional metal film standard sheets can be used for measurement) and high-precision X / Y / Z / Rz translation stages, achieving high resolution at low cost.

[0132] In one embodiment, the beam deflection module 173 may include any one of the following: a Piezo piezoelectric deflector, a micro-electromechanical system (MEMS) deflector, a galvanometer, an electric reflector adjustment mechanism, or a photoacoustic / electro-optic deflector.

[0133] For example, a high-precision beam deflection module 173 is used, and the optical angular resolution of the high-precision beam deflection module 173 is typically less than 0.5 μrad. In one embodiment, a beam deflection module with an optical angular resolution of 0.5 μrad is selected. In this case, tan(θ) can be approximated as θ, and the distance f from the lens center to the sample surface is 20 mm. Then, the resolution Δd of the light spot movement on the sample surface is:

[0134] Δd=tan(θ)×f

[0135] Substituting the values, we get Δd = 10nm. Therefore, by adjusting the deflection of the excitation beam and / or probe beam through the beam deflection module 173, the optical deflection angle θ of the excitation beam and / or probe beam relative to the lens is changed, thereby allowing for very precise adjustment of the relative positions of the two light spot centers on the focal plane. It is evident that in this embodiment of the present disclosure, during the above adjustment process, the resolution of the relative movement between the two light spots can reach at least 10nm; therefore, the accuracy of the effective light spot size can also reach 10nm.

[0136] like Figure 16 As shown, the beam deflection module 173 is set on the excitation beam path. At this time, the position of the probe beam spot is fixed and does not move, while the excitation beam spot on the sample surface is moved by the beam deflection module 173.

[0137] In another embodiment of this disclosure, such as Figure 17 The photoacoustic measurement system shown is related to... Figure 16 The difference in the photoacoustic measurement system shown is that the beam deflection module 181 is positioned on the probe light path between the beam splitter 182 and the beam combiner 183. In this case, the excitation light spot remains fixed, while the probe light spot on the sample surface is moved by the beam deflection module 181.

[0138] In another embodiment of this disclosure, such as Figure 18 The photoacoustic measurement system shown is related to... Figure 17The difference in the photoacoustic measurement system shown is that a first beam deflection module 193 is set in the excitation beam path between the beam splitter 191 and the beam combiner 192, and a second beam deflection module 194 is set in the probe beam path. In this case, the tilt angle of either the first beam deflection module 193 or the second beam deflection module 194 can be controlled independently. Specifically, the first beam deflection module 193 can adjust the position of the excitation beam spot on the sample surface, and the position of the probe beam spot can be adjusted by adjusting the tilt angle of the second beam deflection module 194.

[0139] In this disclosed embodiment, with Figure 18 compared to, Figure 19 The photoacoustic measurement system also includes:

[0140] A chopper 211 is disposed in the excitation light optical path between beam splitter 222 and beam combiner 223, and is used to receive the excitation light to modulate its intensity and frequency. The chopper 211 includes at least one of an optoacoustic modulator or an electro-optic modulator. In this embodiment, compared with mechanical modulation chopping, optoacoustic or electro-optic modulators have a faster response speed (typically on the order of nanoseconds) to modulate changes in the intensity of the excitation light beam, a higher extinction ratio, and better stability. A preferred embodiment in this case is that the chopper is an electro-optic modulator, because compared with optoacoustic modulators, electro-optic modulators have a higher extinction ratio (typically greater than 300:1), more stable modulation intensity, better temperature stability, and better spot quality. Figure 19 The photoacoustic measurement system shown also includes: an optical delay line 221, disposed on the probe light path between the beam splitter 222 and the beam combiner 223, for receiving the probe light to adjust its optical path. For example, as... Figure 19 As shown, a first reflector 224 is provided in the excitation light path to reflect the excitation light to the beam deflection module 225. A second reflector 226 and a third reflector 227 are provided in the probe light path to change the propagation direction of the probe light.

[0141] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for detecting light spots, characterized in that, include: Excitation light and probe light are generated by the light source module. The excitation light and probe light are respectively incident on the sample surface located within the focal depth range of the focusing module through the focusing module along their respective optical paths, forming excitation light spot and probe light spot on the sample surface. The incident direction of the excitation light or the incident direction of the probe light is adjusted by the beam deflection module, so that the excitation light spot and the probe light spot move relative to each other on the sample surface. The relative optical deflection angle between the excitation light and the probe light is obtained in real time. The reflected light of the probe light on the sample surface is collected in real time by the detection module and a signal intensity indicating the reflected light is generated. The adjustment causes the signal intensity to go through a process of increasing and then decreasing from the background noise region value R0 back to the background noise region value R0. Fit the curve showing the relationship between the signal intensity and the relative optical deflection angle; Based on the aforementioned relationship curve, the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target are obtained. Based on the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target, the effective spot size is calculated. The effective spot size is the sum of the size of the excitation light spot and the size of the probe light spot in the direction of relative movement.

2. The spot detection method according to claim 1, characterized in that, The step of obtaining the first target relative optical deflection angle θ1 and the second target relative optical deflection angle θ2 based on the relationship curve includes: Obtain the peak signal intensity Rmax of the relationship curve; Obtain the background noise cutoff value Rmin from the relationship curve; In the relationship curve, the relative optical deflection angle corresponding to the signal strength decreasing from the first side of the peak signal strength Rmax to the background noise cutoff value Rmin is obtained as the first target relative optical deflection angle θ1, and the relative optical deflection angle corresponding to the signal strength decreasing from the second side of the peak signal strength Rmax to the background noise cutoff value Rmin is obtained as the second target relative optical deflection angle θ2.

3. The spot detection method according to claim 2, characterized in that, The background noise cutoff value Rmin satisfies the following relationship: Rmin=(Rmax-R0)*k*(1 / e 2 ), k∈[1, e 2 );or, Rmin = R0.

4. The spot detection method according to claim 1, characterized in that, The calculation of the effective spot size based on the relative optical deflection angle θ1 of the first target and the relative optical deflection angle θ2 of the second target includes calculating the effective spot size using the following formula: D = |θ1 - θ2| * f, where D is the effective spot size and f is the focal length of the focusing module.

5. The spot detection method according to claim 1, characterized in that, Also includes: Before acquiring the signal intensity, adjust the optical path of the excitation light and / or the optical path of the probe light so that the optical path of the excitation light and the optical path of the probe light are equal.

6. The spot detection method according to claim 1, characterized in that, Both the excitation light and the probe light are Gaussian beams, and the signal intensity follows a Gaussian distribution relative to the relative optical deflection angle. A Gaussian function is used to fit the relationship curve of the signal intensity with respect to the optical deflection angle. Where a, b and c are real constants obtained from fitting, and a>0, θ represents the relative optical deflection angle, and R(θ) represents the signal intensity.

7. The spot detection method according to claim 6, characterized in that: The direction of the relative movement is the direction of the major axis of the excitation light spot, and the effective spot size is the sum of the major axis of the excitation light spot and the major axis of the detector light spot, wherein the direction of the major axis of the excitation light spot is the same as the direction of the major axis of the detector light spot; or, The direction of the relative movement is the direction of the minor axis of the excitation light spot, and the effective spot size is the sum of the minor axis of the excitation light spot and the minor axis of the probe light spot, wherein the direction of the minor axis of the excitation light spot is the same as the direction of the minor axis of the probe light spot.

8. A focusing method, characterized in that, The focusing method is applied to a photoacoustic measurement system, including: The focal depth of the focusing module of the photoacoustic measurement system is obtained. Within the focal depth range, the position of the sample is adjusted along a direction perpendicular to the sample surface and / or the position of the focusing module is adjusted along the optical axis of the focusing module to change the sample spacing, wherein the sample spacing is the distance between the focusing module and the sample. According to any one of claims 1-7, the effective spot size corresponding to each sample spacing is obtained respectively; Obtain the minimum value among the multiple effective light spot sizes, adjust the current sample spacing to the sample spacing corresponding to the minimum value, and complete the focusing of the photoacoustic measurement system.

9. A focusing method, characterized in that, The focusing method is applied to a photoacoustic measurement system, including: The focal depth of the focusing module of the photoacoustic measurement system is obtained. Within the focal depth range, the position of the sample is adjusted along a direction perpendicular to the sample surface based on a first adjustment spacing and / or the position of the focusing module is adjusted along the optical axis of the focusing module based on the first adjustment spacing to change the sample spacing. The sample spacing is the distance between the focusing module and the sample. According to any one of claims 1-7, the effective spot size corresponding to each sample spacing is obtained to obtain a first set of effective spot sizes. Obtain the minimum value among the first group of effective spot sizes, and adjust the current sample spacing to the sample spacing corresponding to the minimum value; The sample spacing is adjusted based on a second adjustment distance along the direction perpendicular to the sample surface and / or along the optical axis of the focusing module, and the relationship curve corresponding to each sample spacing is obtained according to the spot detection method according to any one of claims 1-7; wherein the second adjustment distance is smaller than the first adjustment distance; The maximum value corresponding to each of the relationship curves is obtained, resulting in multiple maximum values. The maximum value among the multiple maximum values ​​is determined, and the current sample spacing is adjusted to the sample spacing corresponding to the maximum value, thus completing the focusing of the photoacoustic measurement system.

10. A photoacoustic measurement system, characterized in that, include: The light source module is used to generate excitation and probe light; A beam combiner is used to receive the excitation light and the probe light, and to combine the excitation light and the probe light; A beam deflection module is disposed in the excitation light optical path and / or the probe light optical path, and is used to adjust the relative optical deflection angle between the excitation light and the probe light; A focusing module is used to converge the beam after it has been combined by the beam combiner to form an excitation light spot and a probe light spot on the sample surface, and to collect the reflected light of the probe light on the sample surface. The detection module is used to collect the reflected light of the detection light on the sample surface and generate a signal intensity indicating the reflected light. The control module is used to complete the focusing of the photoacoustic measurement system according to the focusing method according to claim 8 or 9; The data processing module outputs the parameters to be measured based on the signal intensity of the reflected light after the focusing is completed.

11. The photoacoustic measurement system according to claim 10, characterized in that, The beam deflection module includes any one of the following: piezoelectric deflection mirror, MEMS deflection mirror, galvanometer mirror, motorized reflector adjustment mechanism, and photoacoustic / electro-optic deflector.

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