KR101666462B1 - Autofocus method and apparatus for wafer scribing - Google Patents

Abstract

The present invention relates to a method and apparatus for performing real-time autofocus for a wafer scribing system. The method and apparatus use polarized light 42 that is transmitted to the surface 50 of the wafer 10 at an indicator angle directly below the objective lens 26 for the scribing laser beam. The light reflected from the wafer is filtered 56 to remove light from the scribing laser beam and focused on the position sensitive device 58 to measure the distance from the objective lens to the wafer surface.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an autofocus method and apparatus for wafer scribing,

The present invention relates to a method and apparatus for scribing an electronic wafer. In particular, the present invention relates to a method and apparatus for performing real-time focusing of a laser beam used to scribe an LED wafer to aid singulation. More particularly, the present invention relates to a method and system for accurately and efficiently detecting the position of the surface of a transparent or translucent LED wafer while the system scribes the wafer to maintain an accurate relationship between the focus of the laser beam and the wafer surface in real- And more particularly,

Electronic devices are often constructed on a substrate or wafer that includes multiple replication devices of a device for easy manufacturing. These devices must be isolated or singulated prior to packaging and sale. One common way to singulate an electronic device is to scribe the wafer using a laser scribing system to prepare for mechanical cleaving along the scribe. Figure 1 shows a wafer 10 housing an electronic device, one of which is designated by 12. Quot; street "14, which is the area between the electronic devices, which is scribed for subsequent mechanical separation between the devices. Exemplary electronic devices manufactured in this manner include light emitting diodes (LEDs). The LEDs are usually fabricated on a wafer made of crystal sapphire or metal, but other materials may be used. Along the manufacturing process, these wafers are then singulated by scraping with a saw or laser and mechanically cleaved to separate the devices.

A laser scribing system uses a laser to scribe a wafer on which semiconductor dice are grown on one surface of the wafer. The wafer is loaded on a horizontal stage. When this stage translates at high speed (typically between 10 mm / s and 100 mm / s), a laser beam impinges on the top surface along a street separating the individual semiconductor dice formed on the top or bottom surface of the wafer. The interaction between the highly focused laser beam and the wafer creates a kerf or groove on the surface, causing the wafer to mechanically clean along the street. The dice on the wafer are then separated so that each die can be used to manufacture one device. An exemplary system for performing such a wafer scribing function is the AccuScribe-AS2000FX manufactured by the assignee of the present invention. The system uses a diode-pumped solid state laser that harmonic frequency shifts to UV wavelengths for scribing light emitting diode (LED) wafers.

Figure 2 shows a schematic view of a wafer scribing system. The laser 20 produces a working laser beam 22 which is shaped by the laser beam optics 24 and which focuses the working laser beam 22 onto the object Is sent on the lens 26, in this case, to the workpiece 32 as a wafer. The objective lens 26 is attached to a gantry 28 and the gantry 28 is attached to a system base 36 that typically includes a large base plate of granite or other dense material. The system base 36 holds an XY chuck 34 that holds the workpiece 32 firmly. The XY chuck 34 programmably moves the wafer under the working laser to form a scribe on the surface as the laser focus spot 30 processes the workpiece from the workpiece 32. The gantry 28, the system base 36 and the XY chuck 34 are arranged in a precise vertical relationship with the workpiece 32 as the workpiece 32 is moved by the XY chuck 34, Cooperating with maintaining the spot 30, continues the cuff of precise size, shape and quality.

For efficient and uniform scribing of the wafer, the laser beam must be focused on a plane close to the wafer top surface. In other words, the distance between the objective lens and the wafer surface has an optimum value. The advantage imposes stringent requirements on wafer surface flatness and wafer thickness uniformity, and if these wafers can not be efficiently processed, they decrease the yield and increase the price. When mounted on a vacuum chuck, the average thickness of sapphire wafers varies from 10 micrometers to 10 micrometers for different wafers, and the surface flatness varies from 15 micrometers on 2-inch (5.08 cm) wafers. Even when mounted on a vacuum chuck, the metal wafer surface can be rolled and can have a surface height difference of up to 150 mu m on a 2-inch wafer. It may be desirable to focus the working laser beam to have a minimum spot size of 10-50 [mu] m near the surface of the wafer, so that the scribed trenches on the wafer surface have the desired width and depth. Condensing the laser into these small spot sizes requires large numerical aperture (NA) lenses, which also quickly defocus the beam above and below the focal spot. As a result, during the scribing, it is desirable to keep the laser spot within ± 5 μm of the upper surface of the wafer, more preferably within ± 2 μm.

A possible solution to this problem is to track the wafer surface during scrolling to detect changes in the relationship between the workpiece and the laser focus spot using autofocus technology. Autofocus technology includes passive and active methods. The manual method quantifies the amount of out-of-focus using video contrast. The active method requires a beam from a light source and uses this beam or image shift to quantify the out-of-focus amount. The active method is much faster than manual methods and can meet real-time requirements for autofocus tracking when the relative speed between the wafer-mounting stage and the UV laser beam is greater than 10 mm / s. One commonly used autofocus method is described in U.S. Patent No. 6,486,457, in which the collimated laser beam passes through the objective lens off-axis and is focused on a surface near the wafer surface. The reflected beam will then be detected by the position sensitive detector through the objective lens for a second time. The reflected beam will shift due to the distance change between the wafer surface and the objective lens, and the position sensitive detector will produce a signal proportional to this shift. This signal is used to adjust and maintain the distance between the wafer surface and the objective lens to realize autofocus tracking. However, this method has a limited capture range for transparent thin wafers, such as sapphire wafers used in LED manufacturing, because both reflections from the top / bottom surfaces of the wafer can be detected by the position sensitive detector to be. Since the bottom surface has non-uniform reflectivity in different areas, the autofocus accuracy will be poor.

A description of another commonly used active autofocus method can be found in U.S. Pat. Nos. 4,363,962 and 5,361,122. Instead of passing through the objective lens, the beam from the autofocus light source is first projected onto the wafer surface using an additional lens, and then further projected onto the position sensitive detector using another additional lens. The beam impinges on the wafer and is reflected at the grazing angle. In this method, the objective lens, the light source, the additional lens and the position sensitive detector have fixed relative positions. Another method involves adjusting the height of the wafer mounting stage or objective lens (and other components attached thereto) to ensure that the wafer surface is in the focal plane of the objective lens. U.S. Pat. No. 5,008,705 uses this method in conjunction with an interferometer. The US patent (5,825, 469) improved the sensitivity of this method by reflecting the beam twice over the wafer surface. US Pat. No. 5,675,140 combines this approach with an astigmatic lens approach, which is described in articles: Automatic focus control: the astigmatic lens approach, Donald K. Cohen, Wing Ho Gee, M. Ludeke, and Julian Lewkowicz, Applied Optics, 23, pp. 565-570, 1984. These references have not addressed the particular requirement that the bottom surface of the wafer can have different reflectivity at different locations.

An additional difficulty in maintaining a fixed relationship between the surface of the substrate and the laser beam spot position is that LEDs and other electronic devices are often fabricated on transparent substrates such as sapphire or glass substrates. This may raise additional problems, since the upper surface of these wafers may be transparent or translucent and may be flat or rough. The bottom surface of the sapphire wafer can have a pattern, so the reflectivity can vary at different locations. In conventional autofocus systems, which rely on reflection from the wafer to measure, a number of signals of variable intensity can be obtained because of this, and these signals can confuse the system, resulting in lower precision measurements You can prevent the system from working together.

Therefore, as the wafer is scribed, the top surface position of the transparent or translucent wafer can be measured in real time, and the reflection from the top and bottom surfaces of the wafer can be changed to accurately mirror the surface of the translucent and transparent wafer A method and an apparatus for detection are required.

It is an object of the present invention to provide a means for measuring a displacement between a working laser beam focus spot and a workpiece to be laser machined by the working laser beam focus spot. It is a further object of the present invention to measure the displacement between a working laser beam focus spot and a workpiece made of a transparent or translucent material such as sapphire. Another object of the present invention is to measure in real time the displacement between the laser beam focus spot and the workpiece.

In order to improve the performance of the LED scribing system and to lower the production cost for the customer, it is possible to use a tracking autofocusing device to cause the laser scribing system to use an objective lens that focuses the working laser beam on the LED wafer surface The distance between the wafer surfaces is controlled while the wafer is translating horizontally. In an embodiment of the invention, the tracking autofocus device consists of a collimated and polarized laser diode beam, sent through a pinhole and a focusing lens. The wavelength of the laser beam used to measure the surface is chosen to be sufficiently short so that the wafer is accurately measured but the spot size is small enough to avoid interference with the radiation emitted from the plasma cloud produced by the working laser beam or the working laser beam .

The laser beam is then transmitted by the prism to the upper surface of the wafer at an indicator angle between 84 and 87 relative to vertical. In addition, the linearly polarized laser beam is arranged (s-polarized) such that the polarization plane is parallel to the wafer surface. With a combination of the landing angle and the polarization direction, most laser beam energy is reflected from the top surface of the wafer, avoiding interference with reflection from the bottom surface of the transparent wafer. This arrangement also maximizes the reflection from the metal substrate, since the s-polarized wave is largely reflected by the metal surface.

When the laser beam is reflected by the upper surface of the wafer, the laser beam is directed to a lens that focuses the laser beam reflected by the prism onto a bandpass filter, which filters out the radiation from the working laser beam frequency filters out, and passes radiation from the laser beam used to measure the surface. This improves the signal-to-noise ratio of the resulting data. From this, the laser beam is projected onto a position sensitive device (PSD) for measuring the position of the laser beam. This information is digitized and passed to the controller, which calculates the height of the wafer from the displacement of the laser beam on the PSD.

The embodiment of the present invention also operates to calculate the height of the wafer surface in real time (meaning that the working laser beam can measure the height while processing the wafer cuff). This allows the laser processing system to periodically update the wafer height measurement. This embodiment can change and measure the displacement while scribing the wafer because it is combined with the control attached to the gantry that can change the displacement between the objective lens and the wafer in real time. This causes the system to scribe wafers that could not otherwise be scribed because the system is out of the flatness required by a system that does not track and adjust the height in real time, thereby increasing manufacturing yield.

In addition, embodiments of the present invention project a measurement laser beam onto a workpiece to project an ellipse much larger than the laser spot size. By projecting the laser beam through a circular pinhole and then projecting the laser beam onto the workpiece at an angle of 84 DEG to 87 DEG, the laser beam forms an elliptical shape on the workpiece. This gives a reflection average for areas larger than the original spot size and averages the parasitic reflections due to contamination on the workpiece, i.e., unexpected features, to make the measurement more robust .

According to the present invention, as the wafer is scribed, the position of the top surface of a transparent or semi-transparent wafer in real time can be measured and the reflection derived from the top and bottom surfaces of the wafer can be changed, A method and an apparatus for accurately detecting the surface of a substrate can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of a conventional conventional wafer comprising an electronic device.
2 is a schematic view of a conventional wafer scribing system;
3 is a schematic view of an autofocus system.
4 is a schematic diagram of a wafer scribing system with an autofocus system;

As described herein, the present invention relates to a method and apparatus for measuring a displacement of a working laser beam, such as a polarized beam having a wavelength selected to measure in real time the displacement between the focal spot of the working laser beam and the workpiece, avoiding interference from the working laser beam or plasma plume The use of the laser beam at the landing angle solves the problems of the prior art. Figure 3 illustrates an embodiment of the present invention. The collimated beam 42 is emitted by the laser diode 40 and subsequently passes through a small circular opening, namely the pinhole 44, the illumination lens 46 and the prism mirror 48. An exemplary laser diode used for this purpose is 0222-002-01, manufactured by Coherent, Inc. (Santa Clara, Calif.), Which operates at 650 nm wavelength with an output power of approximately 1.6 mW. The distance between the aperture 44 and the lens 46 and the distance between the lens 46 and the wafer top surface 50 are approximately twice the focal length of the lens 46. The openings are accordingly projected onto a plane near the wafer top surface 50. The beam collides with the wafer top surface 50 at an indicator angle; This angle of incidence is between 84 ° and 87 °. Most of the beam is reflected from the top surface and then passes through the prism mirror 52, the condenser lens 54 and the bandpass filter 56 to reach the position sensitive detector (PSD) 58 at point 74 do. The band pass filter 56 blocks ambient light including plasma light emission during wafer scribing to improve the signal-to-noise ratio (SNR). The laser diode 40 is aligned to cause the beam to be s-polarized when the beam impinges on the wafer surface. Using an s-polarized beam increases the SNR when the wafer is thin and transparent because less light will be reflected from the bottom surface of the wafer so that most of the light reaching the PSD 58 It would be from the top surface reflection. Due to the large incidence angle, a long elliptical beam is generated on the wafer surface, averaging the reflectivity over a large area. The long elliptical spot on the wafer surface also minimizes measurement errors due to fine patterns or particulate contamination on the top or bottom surface. The distance between the wafer top surface 50 and the lens 54 and the distance between the lens 54 and the PSD 58 is approximately twice the focal length of the lens 54. [ The aperture 44 is consequently projected onto the PSD 58 accordingly. The wafer is mounted on the xy stage (not shown), while the components (40, 42, 44, 46 and 48) constituting the output section 38 and the constituent elements 52, 54, 56, 58 and 60 are mounted on the z-stage. The PSD output is coupled to the position sense amplifier 60 and used in conjunction with a controller (not shown) to form a servo loop for the z-stage. The wafer or optical system or both can be mounted on the z-stage.

Figure 3 illustrates an embodiment of the present invention. The collimated beam 42 is emitted by the laser diode 40 and subsequently passes through a small circular opening, namely the pinhole 44, the illumination lens 46 and the prism mirror 48. An exemplary laser diode used for this purpose is 0222-002-01, manufactured by Coherent, Inc. (Santa Clara, Calif.), Which operates at 650 nm wavelength with an output power of approximately 1.6 mW. The distance between the aperture 44 and the lens 46 and the distance between the lens 46 and the wafer top surface 50 are approximately twice the focal length of the lens 46. The openings are accordingly projected onto a plane near the wafer top surface 50. The beam collides with the wafer top surface 50 at an indicator angle; This angle of incidence is between 84 ° and 87 °. Most of the beam is reflected from the top surface and then passes through the prism mirror 52, the condenser lens 54 and the bandpass filter 56 to reach the position sensitive detector (PSD) 58 at point 74 do. The band pass filter 56 blocks ambient light including plasma light emission during wafer scribing to improve the signal-to-noise ratio (SNR). The laser diode 40 is aligned to cause the beam to be s-polarized when the beam impinges on the wafer surface. Using an s-polarized beam increases the SNR when the wafer is thin and transparent because less light will be reflected from the bottom surface of the wafer so that most of the light reaching the PSD 58 It would be from the top surface reflection. Due to the large incidence angle, a long elliptical beam is generated on the wafer surface, averaging the reflectivity over a large area. The long elliptical spot on the wafer surface also minimizes measurement errors due to fine patterns or particulate contamination on the top or bottom surface. The distance between the wafer top surface 50 and the lens 54 and the distance between the lens 54 and the PSD 58 is approximately twice the focal length of the lens 54. [ The aperture 44 is consequently projected onto the PSD 58 accordingly. The wafer is mounted on the xy stage (not shown), while the components (40, 42, 44, 46 and 48) constituting the output section 38 and the constituent elements 52, 54, 56, 58 and 60 are mounted on the z-stage. The PSD output is coupled to the position sense amplifier 60 and used in conjunction with a controller (not shown) to form a servo loop for the z-stage. The wafer or optical system or both can be mounted on the z-stage.

For best scribing results, a finite offset between the focal plane of the UV objective and the wafer surface can be used. This scribing result is checked to initialize the distance between the objective lens and the wafer surface. The autofocus component of Figure 3 is then adjusted to ensure that the image of the aperture is located directly beneath the objective lens. This image is then projected onto the point 74 of the PSD 58. Under ideal conditions, the beam path follows the solid line in Fig. If the top surface of the wafer being scribed is not flat, the beam path will change as the x-y stage translates. For example, the dashed line 72 in FIG. 3 becomes a beam path due to the variance of the flatness or thickness of the wafer when the upper surface of the workpiece 50 moves to the new position 70. In this case, the distance between the objective lens and the wafer is longer than the optimum distance. The laser beam now shifts from the point 74 of the PSD 58 to the new position 76 so that the PSD 58 produces a signal proportional to the side shift of the laser beam. The signal is amplified, digitized and sent to the z-stage controller to lower the z-stage to restore the optimum distance between the objective lens and the wafer surface. The servo loop formed by the PSD signal feedback and vertical stage controller thus ensures that the distance between the objective lens and the wafer surface is always optimal during stage translation. This ensures the best scribe results across the entire wafer.

It is new to help with sapphire or metal wafer scribing in LED manufacturing using a tracking autofocus system. The system uses the appropriate aperture size, uses a suitable beam polarization, adds a bandpass filter before the PSD, uses a laser diode in a stable output mode, uses an appropriate aperture size, and uses a high resolution side-type PSD, The system becomes simpler and more powerful than the system of. As discussed in the system description, using appropriate beam polarization and adding a bandpass filter before PSD increases the SNR. The laser diode has a stable beam shape. Instead of emitting the laser diode beam directly onto the PSD, the aperture is projected, so a reference arm is not required. The laser diode output and aperture size are chosen to have enough laser power to reach the PSD, allowing the PSD and amplifier to work under optimal conditions during laser LED scribing. The pinhole size is also large enough to project a sufficiently long elliptical spot on the wafer surface such that the PSD signal averages over the area on the wafer to avoid false responses from the dirt on the wafer top surface. The high resolution PSD 58 increases autofocus sensitivity and does not require double reflections on the wafer. Instead of a segmented photodiode PSD, a 2-sided or 3-sided PSD (part number 1L5SP from On-Trak Photonics, Inc.) can be used to simplify system alignment and auto- Increase focus capture range. The focal lengths of the lens 46 and the lens 54 may be different. The distance between the aperture 44 and the lens 46, the distance between the lens 46 and the wafer surface 50, the distance between the wafer surface 50 and the lens 54 and the distance between the lens 54 and the PSD 58, Does not need to be exactly twice the focal length of the lens. Exceeding a few millimeters will not affect autofocus tracking performance, and alignment of the system is not critical accordingly.

4 shows a laser processing system 80 having an autofocus output section 38, an input section 51 and an objective lens 26 all attached to a Z-axis servo 78. Fig. As described above, when the laser beam 42 detects a displacement change between the workpiece 32 and the objective lens 26, the output section 51 sends a signal to a controller (not shown) The Z-axis servo 78 causes the objective lens 26, the output unit 38 and the input unit 51 to move to compensate for the displacement change and restore it to its nominal value, ) And the workpiece 32 are in a desired relationship.

By appropriately setting the gain and bandwidth of the servo loop formed by the z stage and the PSD signal, the autofocus response can follow a wafer surface height variation of 150 microns across a 2-inch wafer at an xy stage speed of 70 mm / s . The bandwidth of the servo loop is up to 50 Hz for this application. Through a tracking autofocus device, the LED scribing system can track wafer thickness variations of 5-10 μm for 2-inch transparent sapphire wafers with patterns on the bottom surface. The bandwidth of the servo loop is up to 5 Hz for this application. Fast localized height changes will be ignored at these bandwidths, making the system more robust. For faster x / y stage speeds and different surface height variations, the servo loop can be properly optimized to achieve optimal results.

It will be apparent to those skilled in the art that many details of the above-described embodiments of the invention can be varied without departing from the underlying principles of the invention. The scope of the invention should therefore be determined only by the following claims.

40: laser diode 42: collimated beam
44: circular aperture (pinhole) 46: illumination lens
48: prism mirror 50: wafer upper surface
54: condenser lens 56: bandpass filter
58: PSD 78: Z-axis servo

Claims (15)

A method for quantifying the relative displacement between a laser focus spot and a workpiece in a laser processing system, the laser processing system comprising: a working laser generating a working laser beam having a focal spot at a working laser wavelength; Wherein the measuring laser beam and the laser beam detector are operative to quantify the displacement between the laser focal spot and the workpiece, the workpiece having an upper surface and a lower surface, A method for quantifying relative displacement comprising a bottom surface,
Setting the polarization of the measuring laser beam with respect to the upper surface of the workpiece in a particular direction with a specific type of polarization;
A grazing point being selected to maximize the ratio of the measured laser beam energy reflected from the upper surface of the workpiece to the energy amount of the measured laser beam reflected from the bottom surface of the workpiece and subsequently detected by the laser beam detector; angle, and sending the measuring laser beam through the circular opening to the workpiece; And
And sending the working laser beam through the lens to the workpiece to process the workpiece while the measuring laser beam is being sent to the workpiece;
Wherein the working laser and the measuring laser operate at different wavelengths, the measuring laser beam passing through a band-pass filter for filtering the working laser wavelength and plasma light emission caused by the laser treatment of the workpiece;
Wherein the measuring laser beam and the laser beam detector quantify the relative displacement between the laser focus spot and the workpiece while the workpiece is moving relative to the laser focus spot,
Wherein a detector signal from the laser beam detector associated with the relative displacement between the laser focus spot and the workpiece is used to determine a distance between the lens and the workpiece to maintain a desired relationship between the laser focus spot and the workpiece Used to adjust,
A method of quantifying relative displacement. The method of claim 1, wherein the polarization of the particular type of measurement laser beam is linear. The method of claim 1, wherein the specific direction of the measuring laser beam is s-polarized relative to the top surface of the workpiece. 4. A method according to any one of claims 1 to 3, wherein the angle of the land is between 84 and 87 relative to the vertical of the upper surface of the workpiece. The method of claim 1, wherein the movement is greater than 10 mm / s and less than 1000 mm / s. The method of claim 1, wherein the measuring laser comprises a laser diode and operates at a wavelength of less than 700 nm. 4. A method according to any one of claims 1 to 3, wherein the landing angle is selected to measure in real time the displacement between the working laser beam focus spot and the workpiece. 4. The method of any one of claims 1 to 3, wherein the detector comprises a non-segmented position sensitive detector. An apparatus for quantifying relative displacement between a laser focus spot and a workpiece in a laser processing system, said laser processing system comprising: a working laser operative to produce a working laser beam having a laser focus spot at a working laser wavelength; A measurement laser and a measurement laser optics operative to produce a measurement laser beam at a wavelength, and a laser beam detector, wherein the measurement laser beam, the measurement laser optics and the laser beam detector are operative to: The work being operative to quantify the displacement between water, the work comprising an upper surface and a bottom surface, the apparatus comprising:
The measuring laser and the measuring laser optics being operative to send the measuring laser beam through a circular aperture to be reflected from the workpiece and detected by the laser beam detector to quantify the relative displacement between the laser focus spot and the workpiece Wherein the measuring laser beam has a specific type of polarization and the measuring laser beam is sent to impinge on the upper surface of the workpiece in a direction specific to the upper surface of the workpiece;
The measuring laser beam is selected so as to maximize the ratio of the energy of the laser beam reflected from the upper surface of the workpiece to the amount of energy of the laser beam reflected by the bottom surface of the workpiece and subsequently detected by the laser beam detector At an indicator angle, is also sent to the workpiece;
The working laser beam being operative to send the workpiece through a lens for processing the workpiece while the measuring laser beam is being sent to the workpiece;
Wherein the working laser and the measuring laser operate at different wavelengths;
The measuring laser beam being operative with respect to a direction through a band-pass filter operative to filter a working laser wavelength and a plasma light emission caused by a laser treatment of the workpiece;
Wherein the measuring laser beam is transmitted and reflected in the workpiece to quantify the relative displacement between the laser focus spot and the workpiece while the workpiece is rapidly moving relative to the laser focus spot,
Wherein a detector signal from the laser beam detector associated with the relative displacement between the laser focus spot and the workpiece is used to determine a distance between the lens and the workpiece to maintain a desired relationship between the laser focus spot and the workpiece Operating to adjust,
A device for quantifying relative displacement. 10. The apparatus of claim 9, wherein the polarization of the particular type of measurement laser beam is linear. 10. The apparatus of claim 9, wherein the particular orientation is s-polarized relative to the top surface of the workpiece. 10. The apparatus of claim 9, wherein the ground angle is between 84 and 87 relative to the vertical of the top surface of the workpiece. 10. The apparatus of claim 9, wherein the movement is between 10 mm / s and 1000 mm / s. 13. A method according to claim 9 or claim 12, wherein a lens is mounted on a servo loop formed by the z stage and the z stage, and wherein the detector signal is operable to track a 5-10 mu m variation in thickness of the workpiece, / RTI > 10. The apparatus of claim 9, wherein the measuring laser comprises a laser diode and operates at a wavelength of less than 700 nm.

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