AUTOFOCUS METHOD AND APPARATUS FOR WAFER SCRIBING
Technical Field
[0001] This invention relates to methods and apparatus for scribing electronic wafers. In particular it relates to methods and apparatus for performing real time focusing of laser beams used to scribe LED wafers to assist in singulation. More particularly it relates to methods and apparatus for accurately and efficiently detecting the location of the surface of a transparent or semi-transparent LED wafer while the system is scribing the wafer in order to maintain the correct relationship between the focal point of the laser beam and the surface of the wafer in real time.
Background
[0002] Electronic devices are commonly constructed on substrates or wafers containing multiple copies of the device for ease of manufacturing. These devices need to be separated or singulated prior to packaging and sale. One typical method of singulating electronic devices is to use a laser scribing system to scribe the wafer and thereby prepare it for mechanical cleaving along the scribes. Fig. 1 shows a wafer 10 holding electronic devices, one of which is indicated 12. Also indicated is one example of a "street" 14, the areas between electronic devices that are scribed for subsequent mechanical separation of the devices from one another. Exemplary electronic devices manufactured in this fashion include light emitting diodes (LEDs). LEDs are typically manufactured on wafers made of crystalline sapphire or metal, although other materials can be used. Following manufacture, these wafers are then singulated by scribing, either with a mechanical saw or laser, then mechanically cleaved to separate the devices.
[0003] Laser scribing systems use lasers to scribe wafers with semiconductor dice grown on one surface of the wafer. The wafer is loaded onto a horizontal stage. When the stage is translated at a high speed (typically between 10 mm/s and 100 mm/s), the laser beam hits the top surface along the streets which separate the individual semiconductor dice defined on either the top or the bottom surface of the wafer. Interaction between the strongly focused laser beam and the wafer will create kerfs or grooves on the surface, allowing the wafer to be mechanically broken cleanly along the streets. Dice on the wafer can then be separated and each die can be used to fabricate one device. An exemplary system which performs this wafer scribing function is the AccuScribe AS2000FX, manufactured by the assignee of the instant invention. This system uses a diode-pumped solid state laser harmonically frequency shifted to a UV wavelength to scribe light emitting diode (LED) wafers. [0004] Fig. 2 shows a schematic diagram of a wafer scribing system. A laser 20 creates a working laser beam 22 which is shaped and directed by laser beam optics 24 onto an objective lens 26 which focuses the working laser beam 22 to a laser focal spot 30, which is directed to a workpiece 32, in this case a wafer. The objective lens 26 is attached to a gantry 28, which is attached to the system base 36, which typically includes a large base plate made of granite or other dense material. The system base 36 holds the XY chuck 34 which securely holds the workpiece 32. The XY chuck 34 programmable moves the wafer under the working laser to form the scribes on the surface as the laser focal spot 30 machines material from the workpiece 32. The gantry 28, system base 36, and XY chuck 34 cooperate in holding the laser focal spot 30 in precise vertical relationship with the workpiece 32 as it is moved by the XY chuck 34 to maintain the correct size, shape and quality of kerf.
[0005] For efficient and uniform scribing of the wafers, the laser beam should be focused to a plane close to the top surface of the wafer. In other words, the distance between the objective lens and the wafer surface has an optimal value. This imposes a strict requirement on the wafer surface flatness and the wafer thickness uniformity, lowering the yield and increasing the cost unless these wafers can be processed efficiently. The average thickness of a sapphire wafer varies by up to 10 microns for different wafers and the surface flatness varies by up to 15 microns over a 2-inch wafer when it is mounted to a vacuum chuck. The metal wafer surfaces could be warped even if it is mounted onto a vacuum chuck and could have a surface height difference of up to 150 microns over a 2-inch wafer. It is also desirable to focus the working laser beam to have a minimum spot size of 10-50 microns near the surface of the wafer in order to make the trench scribed into the surface of the wafer the desired width and depth. Focusing the laser down to this small a spot size requires a high numerical aperture (NA) lens which also causes the beam to rapidly defocus above and below the focal spot. As a result, it is desirable to keep the laser spot within ±5 microns or more preferably within ±2 microns of the top surface of the wafer while scribing.
[0006] A possible solution to this problem is to track the surface of the wafer while scribing to detect changes in the relationship between the workpiece and the laser focal spot using autofocus techniques. Autofocus techniques include passive methods and active methods. The former uses image contrast to quantify the amount of out-of-focus. The latter needs a beam out of a light source and uses the shift of the beam or the image to quantify the amount of out-of-focus. The active methods are much faster than passive methods and can satisfy the real-time requirement for tracking autofocus when the relative speed between the wafer- mounting stage and the UV laser beam is higher than 10 mm/s. One commonly- used active autofocus method is described by U.S. Patent No. 6,486,457, in which a collimated laser beam passes the objective lens off-axis and is focused to a plane near the wafer surface. The reflected beam will then pass through the objective lens for a second time and be detected by a position sensitive detector. A change of the distance between the wafer surface and the objective lens will cause the reflected beam to shift and the position sensitive detector will yield a signal proportional to the shift. This signal can be used to adjust the distance between the wafer surface and the objective lens and ensure it is constant, thus tracking auto focus is realized. However this method has a limited capture range for transparent thin wafers such as sapphire wafers used in LED fabrication because the reflections from the top and bottom surfaces of the wafers may both be detected by the position sensitive detector. If the bottom surface has uneven reflectivity in different regions, the accuracy of auto focus will be poor.
[0007] Description of another commonly-used active autofocus method can be found in U.S. Patent Nos. 4,363,962 and 5,361 ,122. Instead of passing the objective lens, the beam out of the autofocus light source is first projected onto the wafer surface using an extra lens and then further projected onto a position sensitive detector using another extra lens. The beam hits the wafer and gets reflected at a grazing angle. In this method the objective lens, the light source, the extra lenses, and the position sensitive detector have fixed relative positions. Another methods involves the height of either the wafer mounting stage or the objective lens (and the other components attached to it) being adjusted to ensure that the wafer surface is on the focal plane of the objective lens. U.S. Patent No. 5,008,705 uses this method together with interferometry. U.S. Patent No. 5,825,469 improved the sensitivity of this method by reflecting the beam twice on the wafer surface. U.S. Patent No. 5,675,140 combines this method with the astigmatic lens approach, which was described by a journal paper: 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 did not address the special requirement that the bottom surface of the wafer may have different reflectivity at different locations.
[0008] A further difficulty in maintaining a fixed relationship between laser beam spot location and the surface of a substrate is that LEDs and other electronic devices are sometimes manufactured on transparent substrates such as sapphire or glass substrates. This can provide additional issues since the top surface of these wafers can be either transparent or semi-transparent, and can be either smooth or rough. The bottom surface of the sapphire wafers may have patterns and the reflectivity may vary at different locations. For prior art autofocus systems that rely on reflections from a wafer to make measurements, this may result in multiple signals of varying strength that could confuse the system and result in lower accuracy measurements or prevent the system from working altogether. [0009] What is needed, therefore, is a method and an apparatus for measuring the location of the top surface of transparent or semitransparent wafers in real time as the wafer is being scribed that accurately detects the surface of semitransparent and transparent wafers without being confused by varying reflections coming from both the top and bottom surfaces of the wafer.
Summary of the Invention
[0010] One goal of the instant invention is to provide a means for measuring the displacement between a working laser beam focal spot and a workpiece being laser machined by the working laser beam focal spot. Another goal of the instant invention is to measure the displacement between a working laser beam focal spot and a workpiece where the workpiece is made of a transparent or semi-transparent material such as sapphire. A further goal of the instant invention is to measure the displacement between a laser beam focal spot and a workpiece in real time. [0011] In order to improve the performance of the LED scribing system and lower the unit production cost for the customer, a tracking autofocus device is employed to allow a laser scribing system to control the distance between the objective lens which focuses the working laser beam onto the surface of the LED wafer and the wafer surface while the wafer is being horizontally translated. In an embodiment of the instant invention, the tracking autofocus device consists of a collimated, polarized laser diode beam directed through a pinhole and focusing lens. The wavelength of the laser beam used to measure the surface is selected to be short enough to facilitate spot sizes small enough to accurately measure the wafer, but avoid interference with the working laser beam or radiation emitted from the plasma cloud created by the working laser beam.
[0012] The laser beam is then directed by means of a prism to the top surface of a wafer at a grazing angle of between 84 and 87 degrees from vertical. In addition, the linearly polarized laser beam is arranged so that the polarization plane is parallel to the surface of the wafer (s-polarized). The combination of grazing angle and polarization direction causes the majority of the laser beam energy to be reflected from the top surface of the wafer and thereby avoiding interfering reflections from the bottom surfaces of the transparent wafer. This arrangement also maximizes reflections from metallic substrates since s-polarized waves are highly reflected by metallic surfaces.
[0013] Once the laser beam is reflected by the top surface of the wafer, it is directed by a prism to a lens which focuses reflected laser beam onto a bandpass filter that filters out radiation from the working laser beam frequencies and passes the radiation from the laser beam used to measure the surface. This improves the signal to noise ratio of the resulting data. From there it is projected onto a position sensitive device (PSD) that measures the location of the laser beam. This information is digitized and passed to a controller that calculates the height of the wafer from the displacement of the laser beam on the PSD. [0014] An embodiment of the instant invention is also operative to calculate the height of the wafer surface in real time, meaning that the height can be measured while the working laser beam is machining a kerf in the wafer. This allows the laser processing system to periodically update the wafer height measurement. Coupled with controls attached to the gantry able to change the displacement between the objective lens and the wafer in real time, this embodiment is able to measure the displacement and change it while the wafer is being scribed. This allows the system to scribe wafers that otherwise could not be scribed since they fall outside the flatness required by a system that does not track and adjust height in real time, thereby increasing manufacturing yields.
[0015] In addition, an embodiment of the instant invention projects the measurement laser beam onto the workpiece so as 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 a grazing angle of 84 to 87 degrees, the laser beam forms an elliptical shape on the workpiece. This averages the reflection over a larger area than the original spot size thereby averaging out spurious reflections caused by contamination or unexpected features on the workpiece, thereby making the measurement more robust.
Brief Description of the Drawings
[0016] Fig. 1 is a schematic diagram of a typical prior art wafer containing electronic devices.
[0017] Fig. 2 is a schematic diagram of a prior art wafer scribing system. [0018] Fig. 3 is a schematic diagram of an autofocus system. [0019] Fig. 4 is a schematic diagram of a wafer scribing system with an autofocus system.
Detailed Description of the Preferred Embodiments
[0020] As will be described herein, the instant invention solves the problems of the prior art by employing a polarized, grazing angle laser beam of a wavelength selected to avoid interference from the working laser beam or the plasma plume to measure the displacement between the working laser beam focal spot and the workpiece in real time. Fig. 3 shows an embodiment of the instant invention. A collimated beam 42 is emitted by a laser diode 40 and subsequently passes a small circular aperture or pinhole 44, an illumination lens 46 and a prism mirror 48. An exemplary laser diode used for this purpose is the 0222-002-01 manufactured by Coherent, Inc., Santa Clara, California, and operating at a wavelength of 650 nm at a power of about 1.6 milliwatts. The distance between the aperture 44 and the lens 46, and the distance between the lens 46 and the wafer top surface 50 are about twice the focal length of lens 46. The aperture is thereby imaged to a plane near the wafer top surface 50. The beam hits the wafer top surface 50 at a grazing angle; the incidence angle is between 84 and 87 degrees. Most of the beam is reflected from the top surface and subsequently passing a prism mirror 52, a light collection lens 54, and a band-pass filter 56, then reaching a position sensitive detector (PSD) 58 at point 74. The band-pass filter 56 screens ambient light including plasma light emission during wafer scribing and thus improves the signal to noise ratio (SNR). The laser diode 40 is aligned to ensure the beam is s-polarized when the beam hits the wafer surface. Using 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, thus the majority of the light reaching the PSD 58 thus will come from the top surface reflection. The large incidence angle results in a long elliptical beam on the wafer surface, thus averages the reflectivity over a large area. The long elliptical spot on the wafer surface also tends to minimize errors in measurement caused by fine patterns or particulate contamination on the top or bottom surfaces. The distance between the wafer top surface 50 and the lens 54 and the distance between the lens 54 and the PSD 58 are about twice the focal length of lens 54. The aperture 44 is thereby finally imaged on the PSD 58. The wafer is mounted on an x-y stage (not shown) while the components 40, 42, 44, 46 and 48, which comprise the output section 38 and components 52, 54, 56, 58 and 60 which comprise the input section 51 are mounted on a z-stage. The PSD output is connected to a position sensing amplifier 60, and is then used to form a servo loop for the z-stage in conjunction with a controller (not shown). Either the wafer or the optical systems or both may be mounted on a z-stage.
[0021] Fig. 3 shows an embodiment of the instant invention. A collimated beam 42 is emitted by a laser diode 40 and subsequently passes a small circular aperture or pinhole 44, an illumination lens 46 and a prism mirror 48. An exemplary laser diode used for this purpose is the 0222-002-01 manufactured by Coherent, Inc., Santa Clara, California, and operating at a wavelength of 650 nm at a power of about 1.6 milliwatts. The distance between the aperture 44 and the lens 46, and the distance between the lens 46 and the wafer top surface 50 are about twice the focal length of lens 46. The aperture is thereby imaged to a plane near the wafer top surface 50. The beam hits the wafer top surface 50 at a grazing angle; the incidence angle is between 84 and 87 degrees. Most of the beam is reflected from the top surface and subsequently passing a prism mirror 52, a light collection lens 54, and a band-pass filter 56, then reaching a position sensitive detector (PSD) 58 at point 74. The band-pass filter 56 screens ambient light including plasma light emission during wafer scribing and thus improves the signal to noise ratio (SNR). The laser diode 40 is aligned to ensure the beam is s-polarized when the beam hits the wafer surface. Using 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, thus the majority of the light reaching the PSD 58 thus will come from the top surface reflection. The large incidence angle results in a long elliptical beam on the wafer surface, thus averages the reflectivity over a large area. The long elliptical spot on the wafer surface also tends to minimize errors in measurement caused by fine patterns or particulate contamination on the top or bottom surfaces. The distance between the wafer top surface 50 and the lens 54 and the distance between the lens 54 and the PSD 58 are about twice the focal length of lens 54. The aperture 44 is thereby finally imaged on the PSD 58. The wafer is mounted on an x-y stage (not shown) while the components 40, 42, 44, 46 and 48, which comprise the output section 38 and components 52, 54, 56, 58 and 60, which comprise the input section 51 are mounted on a z-stage. The PSD output is connected to a position sensing amplifier 60, and is then used to form a servo loop for the z-stage in conjunction with a controller (not shown). Either the wafer or the optical systems or both may be mounted on a z-stage.
[0022] For best scribing results, a finite offset between the focal plane of the UV objective lens and the wafer surface may be used. The scribing results are checked to initialize the distance between the objective lens and the wafer surface. The autofocus components in Fig. 3 are then adjusted to ensure that the image of the aperture is located right below the objective lens. This image is then projected to a point 74 of the PSD 58. Under the ideal condition the beam path follows the solid lines in Fig. 3. If the top surface of the wafer being scribed is not flat, the beam path will change when the x-y stage is translated. For example, the dotted lines 72 in Fig. 3 are the beam path when the top surface of the workpiece 50 moves to a new position 70 as a result of variance in the flatness or thickness of the wafer. In this case the distance between the objective lens and the wafer is longer than the optimal distance. The laser beam will now shift away from the point 74 of the PSD 58 to a new position 76 and the PSD 58 yields a signal proportional to the lateral shift of the laser beam. This signal is amplified, digitized and sent to the z-stage controller to bring the z-stage down and restore the optimal distance between the objective lens and the wafer surface. The servo loop formed by the PSD signal feedback and the vertical stage controller therefore ensures that the distance between the objective lens and the wafer surface is always optimal during stage translation. This ensures the best scribing results over the whole wafer. [0023] Using the tracking autofocus system to help scribing sapphire or metal wafers for LED fabrication is new. The system is simpler and more robust than prior art systems described above by using appropriate beam polarization, adding a bandpass filter before the PSD, using a laser diode with stable output mode, using an appropriate aperture size and using a high resolution lateral-type PSD. As discussed in the system description, using appropriate beam polarization and adding a band-pass filter before the PSD improves the SNR. The laser diode has a stable beam shape. The aperture is imaged instead of shooting the laser diode beam directly onto the PSD, so no reference arm is needed. The laser diode power and the aperture size are chosen to have enough laser power reaching the PSD, ensuring the PSD and amplifier working under the optimal condition during laser LED scribing. The pinhole size is also large enough to project a long enough elliptical spot on the wafer surface, thus the PSD signal is averaged over an area on the wafer, avoiding faulty response from dirt on the wafer top surface. A high-resolution PSD 58 increases the autofocus sensitivity, and double reflection on the wafer is not necessary. Using a Duo-lateral or Tetra-lateral PSD (part # 1 L5SP from On-Trak Photonics, Inc.) instead of a segmented-photodiode PSD simplifies the system alignment and increases the autofocus capture range from a few hundred microns to a few millimeters on the PSD. The focal lengths of lens 46 and lens 54 can 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 need not be exactly twice the focal length of the lenses. Being off by a few millimeters will not affect the performance of tracking auto focus and alignment of the system is thus not critical.
[0024] Fig. 4 shows a laser processing system 80 with autofocus output section 38, input section 51 and objective lens 26 all attached to a Z-axis servo 78. As described above, when the laser beam 42 detects a change in the displacement between the workpiece 32, and the objective lens 26, the output section 51 sends a signal to a controller (not shown) which causes the Z-axis servo 78 to move the objective lens 26, output section 38, and input section 51 to compensate for the change in displacement and restore it to its nominal value, thereby maintaining the desired relationship between the working laser focal spot 30 and the workpiece 32. [0025] By appropriately setting up the gain and bandwidth of the servo loop formed by the z stage and the PSD signal, the autofocus response is able to follow up the wafer surface height variation of 150 μm across a 2-inch wafer at an x-y stage speed of 70 mm/s. The bandwidth of the servo loop is -50 Hz for this application. With the tracking autofocus device the LED scribing system can track down 5~10 microns variation in wafer thickness for a 2-inch transparent sapphire wafer with patterns on the bottom surface. The bandwidth of the servo loop is ~5 Hz for this application. Fast local height changes will be ignored with this bandwidth and makes the system more robust. For faster x/y stage speed and different surface height variations we can optimize the servo loop accordingly and obtain optimal results. [0026] It will be apparent to those of ordinary skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.