Laser pressing device

Abstract

The utility model provides a laser pressing device, and relates to the field of laser hot pressing bonding. In the device, a pressure regulating assembly comprises a corrugated pipe, an upper flange plate is arranged at the upper end of the corrugated pipe and sealed with an upper quartz plate, and a lower flange plate is arranged at the lower end of the corrugated pipe and sealed with a lower quartz plate; one of the upper flange plate and the lower flange plate is provided with an air inlet nozzle and an air inlet channel, and the other one is provided with an exhaust nozzle and an air outlet channel; the transmission shaft is of a hollow structure and is coaxially and fixedly connected with the lower end of the pressure adjusting assembly. A displacement sensor of the measuring assembly is used for measuring the expansion amount of the corrugated pipe when air pressure changes, and a force sensor is fixedly installed at the lower end of the transmission shaft and used for measuring bonding pressure. A flexible telescopic device with variable length is formed by the device and the corrugated pipe, opening and closing of the air inlet nozzle and the exhaust nozzle are adjusted according to data fed back by the displacement sensor and the force sensor, air pressure adjustment in the closed cavity can be achieved, namely rigidity adjustment of the pressure adjusting assembly can be achieved, then precise adjustment and control of bonding pressure can be achieved, and precise output is achieved.

Description

Technical Field

[0001] The utility model relates to the technical field of laser thermal compression bonding, and more specifically, to a laser pressing device. Background Art

[0002] When the flip-chip bonding of a chip (wafer) and a substrate (wafer) is carried out by the traditional large-scale thermal reflow technology, due to the different coefficients of thermal expansion of the chip and the substrate, the bonding positions of the chip and the wafer are prone to relative movement during the process, resulting in warping and even bonding failure. Thermal compression bonding refers to a process method in which the wafers are activated and then heated and pressurized in a vacuum chamber to achieve bonding. Since this method requires heating, the efficiency is relatively low.

[0003] To solve the above problems, a flip-chip bonding technical solution using a laser as a heat source has been proposed. Due to the locality of laser heating and the characteristics of high power density, the thermal stress of the chip and the substrate is relatively lower than that of the thermal reflow solution, resulting in smaller warping of the chip, which is beneficial to the improvement of the packaging yield. During the process, the bonding of the chip and the substrate must be completed piece by piece. While ensuring the alignment accuracy, it is also necessary to meet the control of the bonding pressure. If a direct drive device such as a motor is used, the impact on the bonding surface is relatively large, affecting the bonding quality, that is, the control accuracy of the bonding pressure of the laser pressing device in the prior art is insufficient. Summary of the Utility Model

[0004] The purpose of the utility model is to provide a laser pressing device to solve the technical problem of insufficient control accuracy of the bonding pressure of the laser pressing device in the prior art.

[0005] The laser pressing device provided by the utility model can be installed below the light output axis of the laser; the laser pressing device includes a pressure regulating component, a transmission shaft, and a measuring component;

[0006] The pressure regulating component includes a bellows. The upper end of the bellows is provided with an upper flange and sealed with an upper quartz plate, and the lower end is provided with a lower flange and sealed with a lower quartz plate; one of the upper flange and the lower flange is provided with an air inlet nozzle and an air inlet channel is opened, and the air inlet channel communicates the air inlet nozzle and the closed cavity inside the bellows, and the other is provided with an air outlet nozzle and an air outlet channel is opened, and the air outlet channel communicates the air outlet nozzle and the closed cavity;

[0007] The transmission shaft is of a hollow structure and is coaxially and fixedly connected to the lower end of the pressure regulating component;

[0008] The measuring component includes a displacement sensor and a force sensor. The displacement sensor is used to measure the expansion and contraction amount of the bellows when the air pressure changes; the force sensor is fixedly installed at the lower end of the transmission shaft and is used to measure the bonding pressure.

[0009] Furthermore, the laser pressing device further includes a guiding assembly, the guiding assembly includes a guiding rod, the guiding rod is fixedly connected to a base on the laser pressing equipment, and vertically penetrates through the upper flange and the lower flange at both ends respectively.

[0010] Furthermore, the guiding assembly further includes a fixing frame, and the top end of the guiding rod is fixedly connected to the base through the fixing frame.

[0011] Furthermore, the number of the guiding rods is two, and the two guiding rods are symmetrically arranged on both sides of the corrugated pipe.

[0012] Furthermore, the guiding assembly further includes an air-floating bushing, the air-floating bushing is fixedly connected to the base, and is sleeved outside the transmission shaft and has a set gap with the transmission shaft.

[0013] Furthermore, an outer shell is arranged outside the air-floating bushing, and the air-floating bushing is fixedly connected to the base through the outer shell.

[0014] Furthermore, the pressure regulating assembly further includes a connecting fixing plate, and the connecting fixing plate and the upper flange clamp and fix the upper quartz plate therebetween; the transmission shaft is coaxially and fixedly connected to the lower flange, and clamps and fixes the lower quartz plate therebetween.

[0015] Furthermore, the transmission shaft has a fixedly connected flange portion and a hollow rod portion, the flange portion is located at the top end of the rod portion, the flange portion is coaxially and fixedly connected to the lower flange, and clamps and fixes the lower quartz plate therebetween.

[0016] Furthermore, the measuring assembly further includes a mounting bracket and a measuring reference plate, the displacement sensor is fixedly arranged on the mounting bracket, and the mounting bracket is fixedly connected to the transmission shaft; the measuring reference plate is fixedly arranged on the upper flange and is oppositely arranged with the emitting end of the displacement sensor.

[0017] Furthermore, the measuring assembly further includes a mounting plate, the mounting plate is coaxially and fixedly connected to the lower end of the transmission shaft, and the mounting plate is provided with a light-transmitting portion for allowing the laser transmitted from the hollow cavity of the transmission shaft to pass through; the number of the force sensors is multiple, and the multiple force sensors are installed on the lower surface of the mounting plate around the light-transmitting portion.

[0018] The laser pressing device provided by the present utility model can produce the following beneficial effects:

[0019] The laser bonding device provided by the present utility model has a bellows constituting a flexible telescopic device with variable length. Under the sealing of the upper quartz plate and the lower quartz plate, a relatively enclosed space, namely a closed cavity, is formed inside the bellows; a displacement sensor and a force sensor are respectively used to measure the telescopic amount of the bellows and the bonding pressure. When in use, the laser bonding device is installed below the light-emitting axis of the laser. The laser emitted by the laser sequentially passes through the upper quartz plate, the closed cavity, the lower quartz plate, the hollow cavity of the transmission shaft, etc. According to the measurement data fed back by the displacement sensor and the force sensor, the opening and closing of the air inlet nozzle and the air outlet nozzle are adjusted, so as to realize the air pressure adjustment in the enclosed space, that is, to realize the stiffness adjustment of the pressure regulating component, and further be able to realize the precise control of the bonding pressure and achieve precise force output. That is, the laser bonding device can cooperate with the laser to realize the flip-chip bonding of the chip and can perform precise flexible pressurization. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present utility model or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present utility model. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to the provided drawings.

[0021] Figure 1 It is a three-dimensional structure schematic diagram of the laser bonding device provided by the embodiment of the present utility model;

[0022] Figure 2 It is a front view structure schematic diagram of the laser bonding device provided by the embodiment of the present utility model;

[0023] Figure 3 It is a longitudinal sectional structure schematic diagram of the laser bonding device provided by the embodiment of the present utility model;

[0024] Figure 4 It is a partial structure schematic diagram of a laser bonding device including the laser bonding device provided by the embodiment of the present utility model.

[0025] DESCRIPTION OF THE REFERENCE NUMERALS IN THE DRAWINGS:

[0026] 100 - Pressure regulating component; 110 - Bellows; 111 - Upper flange; 112 - Lower flange; 120 - Upper quartz plate; 130 - Lower quartz plate; 140 - Air inlet nozzle; 150 - Air outlet nozzle; 160 - Connecting fixing plate;

[0027] 200 - Transmission shaft; 210 - Flange part; 220 - Rod part;

[0028] 310 - Displacement sensor; 320 - Mounting frame; 330 - Measurement reference plate; 340 - Force sensor; 350 - Mounting plate; 351 - Light-transmitting part;

[0029] 410 - Guide rod; 420 - Fixed bracket; 430 - Air - floating bushing; 440 - Outer shell;

[0030] 500 - Substrate. Detailed implementation manner

[0031] To make the above - mentioned objects, features, and advantages of the present utility model more obvious and understandable, the following will describe in detail the specific embodiments of the present utility model with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present utility model and are not used to limit the present utility model.

[0032] This embodiment provides a laser pressing device, as Figures 1 to 3 shown, and in combination with Figure 4 shown, this laser pressing device is a part of a laser pressing equipment and can be installed below the light - emitting axis of a laser. This laser pressing device includes a pressure - regulating component 100, a transmission shaft 200, and a measuring component. Among them, the pressure - regulating component 100 includes a bellows 110. The upper end of the bellows 110 is provided with an upper flange 111 and sealed with an upper quartz plate 120, and the lower end is provided with a lower flange 112 and sealed with a lower quartz plate 130; the upper flange 111 is provided with an air inlet nozzle 140 and provided with an air inlet channel, and the air inlet channel communicates the air inlet nozzle 140 and the closed cavity inside the bellows 110. The lower flange 112 is provided with an exhaust nozzle 150 and provided with an air outlet channel, and the air outlet channel communicates the exhaust nozzle 150 and the closed cavity; the transmission shaft 200 is a hollow structure and is coaxially and fixedly connected to the lower end of the pressure - regulating component 100; the measuring component includes a displacement sensor 310 and a force sensor 340. The displacement sensor 310 is used to measure the expansion and contraction amount of the bellows 110 when the air pressure changes; the force sensor 340 is fixedly installed at the lower end of the transmission shaft 200 and is used to measure the bonding pressure.

[0033] The laser pressing device provided in this embodiment has a bellows 110 configured as a flexible telescopic device with variable length. Under the sealing of the upper quartz plate 120 and the lower quartz plate 130, a relatively enclosed space, namely a closed cavity, is formed inside the bellows 110; a displacement sensor 310 and a force sensor 340 are respectively used to measure the telescopic amount of the bellows 110 and the bonding pressure. When in use, the laser pressing device is installed below the light-emitting axis of the laser. The laser emitted by the laser sequentially passes through the upper quartz plate 120, the closed cavity, the lower quartz plate 130, the hollow cavity of the transmission shaft 200, etc. According to the measurement data fed back by the displacement sensor 310 and the force sensor 340, the opening and closing of the air inlet nozzle 140 and the air outlet nozzle 150 are adjusted, so as to realize the air pressure regulation in the enclosed space, that is, to realize the stiffness regulation of the pressure regulating component 100, and further to realize the precise control of the bonding pressure, realize precise force output, and while meeting the requirements of force control and position control accuracy, the chip and the substrate will not be damaged. That is, the laser pressing device can cooperate with the laser to realize the flip-chip bonding of the chip and can perform precise flexible pressurization.

[0034] It should be noted that in other embodiments of the present application, the air inlet nozzle 140 and the air inlet channel may also be provided on the lower flange 112, and the air outlet nozzle 150 and the air outlet channel are provided on the upper flange 111, as long as the air pressure in the closed cavity can be regulated.

[0035] Specifically, in this embodiment, as Figure 3 shown, and in combination with Figure 1 and Figure 2 shown, the pressure regulating component 100 further includes a connecting fixing plate 160. The connecting fixing plate 160 and the upper flange 111 clamp the upper quartz plate 120 between them; the transmission shaft 200 is coaxially and fixedly connected to the lower flange 112, and clamps the lower quartz plate 130 between them. In this setting form, the upper quartz plate 120 is clamped by the connecting fixing plate 160 and the upper flange 111, and the lower quartz plate 130 is clamped by the lower flange 112 and the transmission shaft 200. This not only facilitates the operation, but also can ensure the position reliability of the upper quartz plate 120 and the lower quartz plate 130, and further can ensure the tightness of the closed cavity. Of course, in other embodiments of the present application, the upper quartz plate 120 and the lower quartz plate 130 may also be directly blocked at the upper port and the lower port of the bellows 110, that is, at the central holes of the upper flange 111 and the lower flange 112, as long as the position is firm and the two ports of the hollow cavity of the bellows 110 can be blocked.

[0036] More specifically, in this embodiment, as Figure 3As shown, an air inlet groove is formed on the upper surface of the upper flange 111. After the upper quartz plate 120 is sealed to the upper flange 111, the air inlet groove and the lower surface of the upper quartz plate 120 form an air inlet passage; an exhaust groove is formed on the lower surface of the lower flange 112. After the lower quartz plate 130 is sealed to the lower flange 112, the exhaust groove and the upper surface of the lower quartz plate 130 form an exhaust passage. With such a setting, not only are the upper quartz plate 120 and the lower quartz plate 130 relatively simple to set up, but also the processes of the air inlet passage and the exhaust passage are relatively simple.

[0037] Specifically, in this embodiment, as Figure 3 shown, the transmission shaft 200 has a flange portion 210 and a hollow rod portion 220 fixedly connected. The flange portion 210 is located at the top of the rod portion 220. The flange portion 210 is coaxially and fixedly connected to the lower flange 112, and the lower quartz plate 130 is fixedly clamped between the two.

[0038] Specifically, in this embodiment, as Figure 1 and Figure 2 shown, the laser pressing device further includes a guiding assembly. The guiding assembly includes a guiding rod 410. The guiding rod 410 is fixedly connected to the base body 500 on the laser pressing equipment, and vertically penetrates through the upper flange 111 and the lower flange 112 at both ends. Among them, the base body 500 can be a plate member, which is used to fixedly install the parts in the laser pressing device that do not move with the pressure regulating assembly 100, and can drive the entire laser pressing device to lift to transmit the bonding pressure to the machine table for bonding. By setting the guiding rod 410, it is possible to guide the expansion and contraction of the bellows 110 along its axial direction, thereby effectively avoiding the phenomenon of peristalsis when the bellows 110 expands and contracts, and completing the precise alignment and precise force output of the chip.

[0039] Specifically, in this embodiment, continuing as Figure 1 and Figure 2 shown, the number of the guiding rods 410 is two, and the two guiding rods 410 are symmetrically arranged on both sides of the bellows 110. With such a setting, the two guiding rods 410 respectively guide the opposite sides of the bellows 110, which can effectively limit the deformation direction of the pressure regulating assembly 100, so that the pressure regulating assembly 100 does not undergo lateral deformation, torsional deformation, or peristalsis, and the expansion and contraction of the bellows 110 will be smoother and more stable. Here, it should also be noted that in other embodiments of the present application, the number of the guiding rods 410 is not limited to two, but can also be other numbers. For example, the number of the guiding rods 410 can also be three, and the three guiding rods 410 are evenly arranged around the bellows 110.

[0040] Specifically, in this embodiment, the guiding assembly further includes a fixing frame 420. The top end of the guiding rod 410 is fixedly connected to the base body 500 through the fixing frame 420.

[0041] Specifically, in this embodiment, asFigure 3 As shown in the figure, the guiding component further includes an air bearing sleeve 430. The air bearing sleeve 430 is fixedly connected to the base body 500, sleeved outside the transmission shaft 200 and having a set gap with the transmission shaft 200. With such a setting, the transmission shaft 200 can conduct the transmitted pressure to the bonding interface, while the air bearing sleeve 430 enables the transmission shaft 200 to always maintain a vertical movement direction through the air floating effect, thereby ensuring the chip alignment accuracy and bonding accuracy.

[0042] Specifically, in this embodiment, as Figures 1 to 3 shown, an outer shell 440 is arranged outside the air bearing sleeve 430. The air bearing sleeve 430 is fixedly connected to the base body 500 through the outer shell 440. With such a setting, the outer shell 440 provides an installation carrier for the air bearing sleeve 430, plays a protective role at the same time, and also plays a role in connecting it with the base body 500. The air bearing sleeve 430 is fixedly installed in the outer shell 440, and an air film is formed between the air bearing sleeve 430 and the rod portion 220 of the transmission shaft 200, which can prevent the rod portion 220 of the transmission shaft 200 from undergoing flexible deformation exceeding the design requirements under the action of pressure.

[0043] Specifically, in this embodiment, as Figure 1 shown, the measuring component further includes a mounting bracket 320 and a measuring reference plate 330. The displacement sensor 310 is fixedly arranged on the mounting bracket 320, and the mounting bracket 320 is fixedly connected to the transmission shaft 200; the measuring reference plate 330 is fixedly arranged on the upper flange 111 and is oppositely arranged with the transmitting end of the displacement sensor 310. More specifically, the mounting bracket 320 is fixedly installed on the lower surface of the flange portion 210 of the transmission shaft 200. The mounting bracket 320 includes an "L" - shaped plate for installing the displacement sensor 310, and also includes a connecting plate which fixedly connects the "L" - shaped plate and the base body 500. The displacement sensor 310 measures the telescopic amount of the bellows 110 by measuring the distance from the measuring reference plate 330.

[0044] Specifically, in this embodiment, the measuring component further includes a mounting plate 350. The mounting plate 350 is coaxially and fixedly connected to the lower end of the transmission shaft 200, and the mounting plate 350 is provided with a light - transmitting portion 351 for allowing the laser transmitted from the hollow cavity of the transmission shaft 200 to pass through; the number of the force sensors 340 is three. The three force sensors 340 are installed around the light - transmitting portion 351 on the lower surface of the mounting plate 350, and detect the bonding pressure through cooperation with the bonding head. Of course, in other embodiments of the present application, the number of the force sensors 340 is not limited to three, but can also be other numbers, such as four or five, etc.

[0045] More specifically, in this embodiment, the light-transmitting portion 351 is a light-transmitting through-hole. However, in other embodiments of the present application, the light-transmitting portion 351 can also be a material with high laser transmittance, allowing the laser to penetrate and transmit, so that the laser can be directly incident from the top of the device to the lowermost bonding surface to heat the bonding surface. In addition, the corresponding transmission material should also be selected according to the required light intensity requirements. For example, the corresponding quartz material or the corresponding coating should be selected according to the laser attenuation requirements to control the intensity attenuation of the transmitted laser to meet the design requirements.

[0046] In this embodiment, a bonding head is installed below the force sensor 340 of the laser pressing device. The following describes the typical process of adjusting the bonding pressure when using the laser pressing device provided in this embodiment to bond a chip and a substrate:

[0047] Preparation step: The laser pressing device of this embodiment equipped with a bonding head transports and positions the chip to the corresponding position on the wafer under the drive of the driving member.

[0048] Step 1, the bonding head slowly presses down until the force sensor 340 detects the pressure, and at the same time the displacement sensor 310 detects the displacement change. At this time, the exhaust nozzle 150 is closed and the intake nozzle 140 is opened. Compressed air enters the closed space of the bellows 110. Under the action of the air pressure, the bellows 110 deforms downward along the guide rod 410, and the pressure value detected by the force sensor 340 gradually increases.

[0049] Step 2, the force sensor 340 transmits the detected pressure value to the host computer. The host computer compares the collected pressure value with the set value. If the difference between the two values is within the required range, the intake nozzle 140 is closed, and at the same time the exhaust nozzle 150 remains closed.

[0050] Step 3, after the pressure value reaches a stable state, the laser heats the bonding surface.

[0051] Step 4, as the bonding surface is heated by the laser, the contacts on the bonding surface gradually melt. At this time, the displacement sensor 310 detects a sudden increase in the displacement value and transmits the detected value to the host computer. The host computer triggers the displacement control mode. At this time, the exhaust nozzle 150 is opened to release the gas in the bellows 110 so that the bonding head does not undergo a large displacement change.

[0052] Step 5, after the laser is turned off, the host computer enters the force control mode, and judges the opening and closing of the intake nozzle 140 and the exhaust nozzle 150 through the force value transmitted by the force sensor 340 to stabilize the air pressure in the bellows 110 within the set range. At the same time, the bonding head is lifted to enter the next cycle.

[0053] Finally, it should also be noted that in this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements not only includes those elements but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0054] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present utility model. Various modifications to the embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present utility model. Therefore, the present utility model will not be limited to the embodiments shown herein, but rather should be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A laser pressing device, characterized in that: Can be installed below the light output axis of the laser; the laser pressing device comprises: A pressure regulating assembly (100) comprises a bellows (110), wherein the upper end of the bellows (110) is provided with an upper flange (111) and is sealed with an upper quartz plate (120), and the lower end of the bellows (110) is provided with a lower flange (112) and is sealed with a lower quartz plate (130); one of the upper flange (111) and the lower flange (112) is provided with an air inlet nozzle (140) and an air inlet passage, wherein the air inlet passage connects the air inlet nozzle (140) and a closed cavity in the bellows (110); the other of the bellows (110) is provided with an air outlet nozzle (150) and is provided with an air outlet passage, wherein the air outlet passage connects the air outlet nozzle (150) and the closed cavity; A transmission shaft (200), the transmission shaft (200) being a hollow structure and being coaxially fixedly connected to the lower end of the voltage regulating assembly (100); The measuring assembly comprises a displacement sensor (310) and a force sensor (340), wherein the displacement sensor (310) is used to measure the expansion and contraction amount of the bellows (110) when the air pressure changes; and the force sensor (340) is fixedly mounted on the lower end of the transmission shaft (200) and is used to measure the bonding pressure.

2. The laser pressing device according to claim 1, characterized in that: The laser pressing device also includes a guide assembly, which includes a guide rod (410). The guide rod (410) is fixedly connected to a base (500) on the laser pressing device, and its two ends respectively vertically penetrate the upper flange (111) and the lower flange (112).

3. The laser pressing device according to claim 2, characterized in that: The guide assembly further comprises a fixing frame (420), and the top end of the guide rod (410) is fixedly connected to the base (500) via the fixing frame (420).

4. The laser pressing device according to claim 2 or 3, characterized in that: The number of the guide rods (410) is two, and the two guide rods (410) are symmetrically arranged on both sides of the corrugated tube (110).

5. The laser pressing device according to claim 2, characterized in that: The guide assembly further comprises an air-floating sleeve (430), wherein the air-floating sleeve (430) is fixedly connected to the base (500) and is sleeved outside the transmission shaft (200) and has a set gap with the transmission shaft (200).

6. The laser pressing device according to claim 5, characterized in that: The air-floating sleeve (430) is provided with a shell (440) outside, and the air-floating sleeve (430) is fixedly connected to the base (500) via the shell (440).

7. The laser pressing device according to claim 1, characterized in that: The voltage regulating assembly (100) further comprises a connecting and fixing plate (160), wherein the connecting and fixing plate (160) and the upper flange (111) fix the upper quartz plate (120) therebetween; The transmission shaft (200) is coaxially fixedly connected to the lower flange (112), and the lower quartz plate (130) is fixedly clamped between the two.

8. The laser pressing device according to claim 7, characterized in that: The transmission shaft (200) comprises a fixedly connected flange portion (210) and a hollow rod portion (220), wherein the flange portion (210) is located at the top end of the rod portion (220), the flange portion (210) is coaxially fixedly connected to the lower flange plate (112), and the lower quartz plate (130) is fixedly clamped therebetween.

9. The laser pressing device according to claim 1, characterized in that: The measuring assembly further comprises a mounting frame (320) and a measuring reference plate (330), the displacement sensor (310) is fixedly arranged on the mounting frame (320), and the mounting frame (320) is fixedly connected to the transmission shaft (200); The measurement reference plate (330) is fixedly disposed on the upper flange (111) and is disposed opposite to the transmitting end of the displacement sensor (310).

10. The laser pressing device according to claim 1, characterized in that: The measuring assembly further comprises a mounting plate (350), wherein the mounting plate (350) is coaxially fixedly connected to the lower end of the transmission shaft (200), and the mounting plate (350) is provided with a light-transmitting portion (351) for allowing laser light transmitted from the hollow cavity of the transmission shaft (200) to pass through; the force sensors (340) are multiple in number, and the multiple force sensors (340) are mounted on the lower surface of the mounting plate (350) around the light-transmitting portion (351).

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