WO2005048340A1

WO2005048340A1 - Vacuum chuck

Info

Publication number: WO2005048340A1
Application number: PCT/IL2004/001040
Authority: WO
Inventors: Gil Shetrit; Ofer Sela
Original assignee: Advanced Dicing Technologies Ltd.
Priority date: 2003-11-16
Filing date: 2004-11-12
Publication date: 2005-05-26
Also published as: IL158890A0

Abstract

A vacuum chuck having a planar upper surface for holding a planar substrate for dicing using a dicing saw comprising a vacuum surface and a light source for illuminating at least an area of said substrate from below.

Description

VACUUM CHUCK FIELD OF THE INVENTION The present invention relates to a vacuum chuck for holding substrates for the microelectronic industry and allowing their dicing into individual components, wherein the chuck includes a novel positioning system using light transmitted through the substrates. The invention also refers to the method of positioning the vacuum chuck with respect to the dicing blade. BACKGROUND OF THE INVENTION Many components for the electronic industry, particularly for the microelectronics industry, are fabricated in large two-dimensional arrays on a substrate or wafer. The manufacturing route of such components typically employs deposition of a plurality of layers, including (i) metallic legs for contacting the component into a circuit, (ii) active layers that give provide the component with its characteristic behaviour and (iii) insulating layers for encapsulation or packaging, that isolate the components from their surroundings, protecting them from atmospheric contamination and from the effects of stray electromagnetic effects when in use. Once fabricated, such substrates, which may be 12" (30 cm) disks having thousands of components thereon, are cut up to separate the individual components from each other. Typically the components are arranged in a rectangular array, and the cutting up is performed by sectioning through the substrate using a dicing blade between each adjacent pairs of rows and columns of components. Individual components may be less than a millimeter square. To separate the individual components without damaging them, it is critical that the dicing blade be correctly positioned with respect to the array of components and that the substrate is securely held during the dicing process. The preferred method of holding such substrates is with a vacuum chuck. During and after dicing, the separated, individual components are tiny, and, if free to move, are liable to get damaged or to damage the dicing saw. Micro-porous vacuum chucks have been developed to securely hold substrates prior to dicing, and to keep the individual components held within a rectangular array throughout the dicing process. Such micro-porous vacuum chucks are made of a micro-porous material, as available from Kyocera® for example. There are a range of such materials available, typically having 30% to 50% open pores, and average pore sizes per plate being in the range of from single microns to 100 if microns. Optical positioning systems have been developed for the correct positioning of the substrate for dicing purposes, that is, for correct alignment of the substrate with respect to the dicing blade. These systems illuminate the upper surface of the substrate, thereby allowing the substrate to be viewed on a screen, using light reflected therefrom. Either manually or automatically by using optical recognition software and automatic micro-positioning motors, the position of the components is determined, and the position of the dicing blade, or the chuck itself, is then adjusted to cut between the components. When using reflected light, the positioning of the substrate for dicing is accomplished with respect to the upper surfaces of the components. The contact layer, which is typically the lowest functional layer of the component, typically extends beyond the upper, active layers of the device, and the dicing is typically designed to cut through the contact layer and to separate the components. There is a problem however, in that the contact layers are typically fabricated from electrical solder, gold or other soft, ductile metal or alloy. The position of these layers is often not as accurately determined as the positioning of the active layers of the component. If the positioning of the dicing blade is determined by the active components only, the blade may miss the solder layer and some of the separated components after dicing lack proper termination and are discarded. However, illuminating lower layers of the multilayered component using reflected light, and using that illumination for imaging purposes is generally unsatisfactory, and poor edge resolution often results. It is critical in today's competitive world to keep manufacturing yields as high as possible, and it is necessary to minimize the wastage of fully fabricated components at the end of long and difficult multi-stage manufacturing routes. Current practices of automated optical inspection using reflected light are wasteful where the accuracy of the printing of the termination layer is poor. It is, however, difficult to accurately print terminations. The present invention is directed to solving this problem of dicing arrays of electronic components.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a vacuum chuck having a planar upper surface for holding a planar substrate for dicing using a dicing saw comprising: a vacuum surface and a light source for illuminating at least an area of the substrate from below.

The vacuum surface may be a grooved vacuum surface, which may be fabricated from a ceramic, a glass, a metal or combinations thereof, for example.

Preferably however, the vacuum surface is a micro-porous vacuum surface, and may be fabricated from a micro-porous ceramic or from micro- porous aluminium, for example.

The light source is preferably an array of LEDs embedded in the chuck and covered with a transparent material to provide said planar upper surface. However, other direct light sources such as discharge tubes, filament bulbs and solid state light emitting components may be used. Similarly, indirect light source such as fiber optic conduits and mirror surfaces for transmitting light originating from a remote light source may be used.

The planar substrate is a transparent substrate on which is deposited a rectangular array of components and dicing involves separating the components from the array, by cutting through the substrate.

Where the planar substrate is silicon the light source may be a source of IR radiation that penetrates silicon. The components may be optical components, electronic components or microelectronic components.

In general, the light source is a source of radiation of electro-magnetic waves to which the substrate is transparent. In a second aspect, the present invention is directed to providing a dicing apparatus comprising a vacuum chuck as claimed in any of claims 1 to 14, and further comprising a light detection means for positioning above a substrate, a dicing blade, and a means for positioning the dicing blade with respect to the substrate. In a third aspect there is provided a method of positioning a transparent substrate with respect to a dicing blade by holding the substrate on a chuck having a micro-porous vacuum chucking surface and a light source therein for illuminating the substrate by transmission, for viewing from above.

The method comprises the steps of: placing substrate on vacuum chuck; activating a vacuum to hold said substrate securely; switching on a light source to illuminate the substrate from below with an electromagnetic radiation; monitoring electromagnetic radiation intensity variation above the substrate; imaging edges of components using an imaging system; detecting extreme edge position of components in array using an appropriate image analysis algorithm; adjusting relative position of a dicing blade with respect to the substrate; dicing the substrate between adjacent rows or columns of components to separate the individual components from their neighbours; releasing vacuum pressure and collecting the components. By micro-porous material, a material having an open porosity, typically between 30%> and 50%> is intended. Such materials are available commercially, from Kyocera® for example, and come in a range of average pore sizes, from single microns to hundreds of microns.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be further understood and appreciated from the following detailed description taken in conjunction with the drawings in which: Fig. 1 is a bird's eye view of the lower contact pads of adjacent microelectronic components, as imaged using light reflected off upper surface of a substrate. Fig. 2 is a view from above of multilayer components showing typical offsets between lower layers and top surfaces of multilayer microelectronic components. Fig. 3 is a view of the contact pads of adjacent components in silhouette, as viewed using transmitted light. Fig. 4 is a schematic representation of a vacuum chuck of the present invention from above. Fig. 5 is an isometric projection of a preferred embodiment of the chuck shown in Fig. 4. Fig. 6 is a schematic illustration of a dicing saw arrangement including the vacuum chuck of the present arrangement. Fig. 7 is a flow diagram illustrating a method of positioning a dicing blade using the vacuum chuck of the present invention.

DETAILED DESCRIPTION OF THE INVENTION To enable the correct positioning of dicing blades with respect to arrays of components mounted on transparent substrates, such as glass for example, it has been found useful to illuminate the substrate from below. In this way, the position of the most extensive layer, which is generally the contact layer, fabricated from a soft metal or alloy, such as solder, copper, aluminium, gold or silver, stands out in sharp contrast to the surroundings, and an automated imaging system may then be used to accurately position the substrate with respect to the dicing blade for dicing purposes. In contradistinction to prior art systems using reflected light for positioning components, as in US 6,032,997 to Elliot, for example, using transmitted light enables the dicing blade to be optimally positioned with respect to the widest layers and not with respect to the uppermost layers. It will be appreciated that using transmitted light is not suitable for positioning dicing blades with respect to components fabricated on opaque substrates such as silicon wafers. However, where the substrate is transparent, such as where made from glass or some clear polymeric material, and where the upper surfaces are not the critical ones with respect to which the dicing should occur, more accurate positioning is obtained using transmitted light. With reference to Fig. 1, there is shown the typical quality of image obtained when attempting to focus on lower layers of components, mounted on transparent substrates and illuminated using reflected light from above. In the image, (which demonstrates a particular example of the technological challenge that the present invention addresses), the lower layers being imaged are contact pads 14, 16 of components 10, 12 in the same column 20 and adjacent rows of a rectangular array, separated by a path 18. Cutting through the substrate along path 18 separates adjacent components 10, 12. In some applications, it may be required to cut along the pathway 18 substantially equidistantly between the edges 15, 17 of the contact pads 14, 16. However, as shown, there may be considerable difficulty in resolving the edges 15, 17 of the contact pads 14, 16. As shown in figure 2 however, which is an image of such a multilayer component showing the edges of the top layers and of the pads, if the clearly resolved upper edges A-A, B-B are used for ascertaining the correct positioning of a dicing blade, such that the blade is positioned midway between upper edges A-A and B-B, a cut will be made along X-X. Where there is an offset of lower layers 14', 16', to cut midway between such layers, the dicing saw should preferably pass along line Y-Y. The offset between X-X and Y-Y may be tens of microns. Particularly where the extent of the offset is unknown, it is not possible to optimally position the dicing blade, and the dicing operation will tend to result in damage to and wastage of components. By way of contrast, with reference to Fig. 3 there is shown the quality of image obtainable using transmitted light, the contact pads 14", 16" appearing in silhouette, in sharp relief with the pathway 18". For the narrow range of applications where opaque lower layers embedded in a transparent substrate are used for positioning of a dicing blade, illumination using transmitted light is a novel solution to the problem of alignment of dicing blade using these lower layers, and a narrow patent is requested for positioning methods and apparatus using this solution. Transparent vacuum chucks per se. are known, and the reader is referred to US Patent Application No. 2002/0109775 to White, for example. In US Patent Application No. 2002/0109775 areas of the chuck are fully transparent and the chuck is illuminated from below. The chuck is designed for positioning printed circuit boards for drilling purposes. Such a system is unsuitable for dicing purposes however, as a rectangular array of holes drilled in a transparent chuck is adequate for holding large work-pieces, but totally unsuitable for retaining individual components such as diced up substrates. With reference to Fig. 4 there is shown schematically, an embodiment of the present invention, being a vacuum chuck 40, fabricated from a micro- porous material 42 in which light emitters are arranged in a cross shaped formation 44 under the surface 46 of the vacuum chuck 40. To ensure planarity, the light emitters are typically covered with a transparent material, such as an epoxy for example, and then the upper surface 46 of the vacuum chuck 40 is then ground and polished to required smoothness tolerances. This novel vacuum chuck 40 is a practical and cost effective solution for positioning transparent substrates with respect to dicing blades for accurate positioning of dicing blades with respect to widest layers rather than upper layers of multilayer components. Since such components are typically arranged in rectangular arrays, the use of a cross-shaped formation 44 is ideal as the blade may be correctly positioned with respect to the center components in each row / column. It will be appreciated that a dicing blade will cut the substrate from one side to the other, between each pair of rows / columns, so a more sophisticated and complex array of illuminating elements are superfluous. Microporous materials 42 are readily available commercially, from

Kyocera for example, and include both micro-porous ceramics and micro- porous aluminium. Available materials typically have an open pore density of 30%) to 50%>, and an appropriate average pore size, which may be from single microns to hundreds of microns. It will be noted, that where the rectangular array of components is diced up but the substrate is not cut through completely, the chuck is not required to be micro-porous, and a grooved chuck may be used. With reference to Fig. 5, in a preferred, relatively low cost embodiment, also suitable for dicing through to separate individual components completely, the vacuum chuck 50 comprises micro-porous ceramic 52, with LEDs 58 arranged into a cross shape 54. The cross shaped tracks are filled in with a transparent epoxy material 60 which holds the LEDs securely, and enables the upper surface 56 of the chuck 50 to be ground smooth. Although LEDs are reliable cheap light emitting components, other light emitting components, such as filament bulbs, discharge tubes, including fluorescent lights, for example, and even other solid state light emitters, such as lasers, for example, may be used. Furthermore, indirect light sources, such as upwardly pointing light conduits embedded into the chuck, including, but not limited to fiber-optic filaments for example, may be used instead. Components fabricated on transparent substrates for which the correct positioning of a dicing blade via transmitted light is suitable, include miniature CCD cameras, for applications such as cellular phones and for medical imaging. However, it will be appreciated, that there are many other optical, electronic and microelectronic components and circuits printed on substantially transparent substrates. Furthermore, the substrates need only be substantially transparent to at least one specific wavelength emitted by the specific light emitters used in a particular application. In other words, providing that an electromagnetic emission from the emitters within the chuck may pass through the substrate in a manner providing contrast due to the components, for detection above the substrate, a dicing blade may be correctly positioned with respect to components, in accordance with the present invention. For example, it will be noted that for dicing silicon substrates, an IR wavelength is suitable, since silicon is transparent to IR but opaque to visible light. With reference to schematic Figure 6, the invention is also directed to a dicing system 60 comprising a vacuum chuck 62 with illumination means 64 therein, as described hereinabove, light detection means 66 for positioning above a substrate 68, a dicing blade 70, and a means for positioning the dicing blade with respect to the substrate, typically for moving either blade 70 or chuck 62 and substrate 68, incrementally along orthogonal axes α-β-γ, and possibly tilt and rotation. The present invention is also directed to a method of positioning a dicing blade with respect to a transparent substrate by holding the substrate on a chuck having a vacuum chucking surface and a light source therein for illuminating the substrate by transmission, for viewing from above, either by eye, or preferably, using an automated imaging system. With reference to Figure 7, the method involves: positioning the substrate on the vacuum chuck (Step 1); activate the vacuum system to hold the substrate securely in place on the chuck (Step 2); adjusting the position of the stage with respect to the cutting blade for alignment (step 3), it being appreciated that this may be done by using a reflected light system from above, as known in the prior art; switching on the light source within the chuck, to illuminate the substrate from below with electromagnetic radiation (Step 4) - as explained hereinabove, typically the light source is an array of LEDS in a cross-shaped configuration, and the electromagnetic radiation is visible light. The electromagnetic radiation intensity variation above substrate is monitored (step 5) - although the eye may be used, with appropriate optics, such as a traveling microscope system, optionally and preferably an imaging system is used for the monitoring, and such a system is preferably computerized. (Imaging systems of this type are widely used with dicing saws and other processing equipment used in the fabrication of components for the microelectronics industry, and, being standard equipment known to the man of the art, will not be discussed further herein). The edges of the components are viewed using the imaging system (step 6), and the extreme edge position of components within the array is determined using appropriate image analysis algorithms (step 7). In this manner, the relative position of the dicing blade with respect to the substrate may be adjusted (step 8) using appropriate micro- positioning means, such as stepper motors on cutting stage, and the like. The substrate is then diced between adjacent pairs of rows or columns of components, to separate the array into individual components (step 9). Then the vacuum pressure may be released (step 10) and the components collected. It will be appreciated that the invention is not limited to what has been described hereinabove merely by way of example. Rather, the invention is limited solely by the claims which follow, in which the word "comprise" and variations thereof, such as "comprising", "comprised" and the like, imply that the listed elements or steps are included, but not generally to the exclusion on other elements or steps.

Claims

1. A vacuum chuck having a planar upper surface for holding a planar substrate for dicing using a dicing saw comprising: a vacuum surface and a light source for illuminating at least an area of said substrate from below.

2. A vacuum chuck as claimed in claim 1 wherein said vacuum surface is a grooved vacuum surface.

3. A vacuum chuck as claimed in claim 2 wherein said grooved vacuum surface is fabricated from a ceramic, a glass, a metal or combinations thereof.

4. A vacuum chuck as claimed in claim 1 wherein said vacuum surface is a micro-porous vacuum surface.

5. A vacuum chuck as claimed in claim 1 wherein said micro-porous vacuum surface is fabricated from a micro-porous material selected from the list of micro-porous ceramics and micro-porous aluminium.

6. A vacuum chuck as claimed in claim 1 wherein said light source is an array of LEDs embedded in the chuck and covered with a transparent material to provide said planar upper surface.

7. A vacuum chuck as claimed in claim 1 wherein said light source is a direct light source selected from the list of discharge tubes, filament bulbs and solid state light emitting components.

8. A vacuum chuck as claimed in claim 1 wherein said light source is an indirect light source selected from the list of fiber optic conduits and mirror surfaces for transmitting light originating from a remote light source.

9. A vacuum chuck as claimed in claim 1 wherein said planar substrate is a transparent substrate on which is deposited a rectangular array of components and said dicing involves separating said components from said array, by cutting through said substrate.

10. A vacuum chuck as claimed in claim 1 wherein said planar substrate is silicon and said light source is a source of IR radiation that penetrates silicon.

11.A vacuum chuck as claimed in claim 9 or claim 10 wherein said components are optical components.

12. A vacuum chuck as claimed in claim 9 or claim 10, wherein said components are electronic components.

13. A vacuum chuck as claimed in claim 9 or claim 10 wherein said components are microelectronic components.

14. A vacuum chuck as claimed in any of claims 1 to 13 wherein said light source is a source of radiation of electro-magnetic waves to which the substrate is transparent.

15. A dicing apparatus comprising a vacuum chuck as claimed in any of claims 1 to 14, and further comprising a light detection means for positioning above a substrate, a dicing blade, and a means for positioning the dicing blade with respect to the substrate.

16. A method of positioning a transparent substrate with respect to a dicing blade by holding said substrate on a chuck having a vacuum chucking surface and a light source therein for illuminating said substrate by transmission, for viewing from above.

17. The method of claim 16 comprising the steps of: placing substrate on vacuum chuck; activating a vacuum to hold said substrate securely; switching on a light source to illuminate the substrate from below with an electromagnetic radiation; monitoring electromagnetic radiation intensity variation above the substrate; imaging edges of components using an imaging system; detecting extreme edge position of components in array using an appropriate image analysis algorithm; adjusting relative position of a dicing blade with respect to the substrate; dicing the substrate between adjacent rows or columns of components to separate the individual components from their neighbours; releasing vacuum pressure and collecting separated components.