1. Field of the Invention
The present invention relates generally to techniques for fabrication of integrated circuit chips, and more particularly, to a method for protection of special-purpose chips, such as image sensors and the like.
2. Description of Related Art
A number of integrated circuit chips are batch processed as an array of chips on a silicon wafer. The wafer is then cut, or diced, with a saw blade or laser beam to separate, or singulate, the individual chips, or dies, from the wafer.
The top surface of a conventional integrated-circuit die has various electrical circuit elements formed thereupon. Conventional integrated-circuit dies often have a protective passivation layer formed over the electrical circuit elements and covering the top surface of the die. Conventionally, oxide areas, scribe lines, or other indicia are provided on the wafer surface to assist in guiding a saw blade through singulation streets between the various dies. Singulation of conventional dies from a wafer includes mounting the bottom surface of the wafer to a saw pallet using, for example, a double-sided adhesive tape such as blue NITTO tape and passing the saw blade through the wafer. This sawing process and other singulation processes produce silicon shards and particulates. The conventional passivation layer protects the underlying circuitry from the shards, particulates, and water used in the singulation process.
For certain applications, the top surface of a die, e.g., a micromachine die, has formed on it a special-purpose area having various types of special-purpose devices, such as, for example, micromachines. It is important that the special-purpose area not be contacted and contaminated with silicon shards and particulates, or water from the singulation process. Otherwise, the special-purpose area is damaged or destroyed. Consequently, the special-purpose device must be protected from undesired contamination or contact during the die-singulating process.
FIG. 1 illustrates one type of prior art technique for protecting a special-purpose area, e.g., containing a micromachine, during a wafer dicing process, as disclosed in a Roberts, Jr. et al., U.S. Pat. No. 5,362,681, which is herein incorporated by reference in its entirety. A wafer 10 of silicon or other suitable material is typically processed to produce a plurality of special-purpose dies, or chips, 12 (hereinafter dies 12). The dies 12 are arranged in a two-dimensional array on the wafer 10 and are separated by singulation streets 14 between adjacent dies 12. Each die 12 includes a special-purpose area 16 that is formed on a top surface 17 of the die 12. Each special-purpose area 16 includes a special-purpose device, such as a micromachine. When the wafer 10 is singulated into separate dies 12, silicon shards and particulates, as well as a silicon and water slurry are produced. Contact with these byproducts of sawing may harm the special-purpose areas 16.
The prior art technique of FIG. 1 uses two separate layers of MYLAR tape for protecting the special-purpose areas 16 from silicon shards, particulates, and slurry during singulation of the dies 12 by sawing of the wafer 10. A first layer 18 of 5 mil (0.13 mm) MYLAR tape has a lower surface 19 that is fixed to the top surface 17 of the dies 12 and more generally of the wafer 10 with a thin layer 20 of adhesive material. If the first layer 18 were to completely cover the top surface 17 of the dies 12, it would permanently adhere to and damage the special-purpose areas 16.
Consequently, the first layer 18 is required to have a set of precisely positioned, spaced-apart punched holes 22 that are mechanically punched into the first layer 18 to precisely accommodate the special-purpose areas 16. The punched holes 22 need to have diameters large enough to provide clearance for the special-purpose areas 16. A second separate layer 24 of 3 mil (0.07 mm) MYLAR tape is fixed to the upper surface 25 of the first layer 18 to seal the top end of the mechanically punched holes 22. The holes 22 in the first layer 18 must be precisely aligned with the aid of video cameras over the special-purpose areas 16 on the wafer 10 prior to attachment of the first layer 18 of MYLAR tape to the top surface 17 of the wafer 10 with the thin layer 20 of adhesive material. After sawing, the layers 18, 24 are removed with tweezers.
It should be readily apparent that mechanically punching the holes 22 in the first layer 18, precisely aligning the holes 22 in the first layer 18 over the special-purpose areas 16 on the wafer 10, and removing the layers 18, 24 with tweezers is more expensive and complex than the prior art technique of simply covering a wafer with a passivation layer during processing of conventional integrated-circuits on the wafer to protect underlying electrical circuitry elements during die sawing.
Consequently, a need exists for a simple, economical technique for protecting special-purpose areas on the surface of a wafer from contaminants during singulation of dies from the wafer.
According to the principles of this invention, a structure is provided for protecting a special-purpose area located on a top surface of a special-purpose die during singulation of the special-purpose die from a wafer, where the special-purpose die has a bottom surface. The structure includes a protective layer of tape having an adhesive lower surface formed by a polymerizable material. The adhesive lower surface of the protective layer of tape is attached to the top surface of the special-purpose die to overlie and/or contact the special-purpose area on the special-purpose die and to protect the special-purpose area during sawing and detachment of the special-purpose die.
In one embodiment, the protective layer of tape has a first polymerized upper zone and a second lower zone that is unpolymerized and that has an adhesive lower surface.
The structure further includes sawing indicia located on a top surface of the wafer and visible through the protective layer of tape to aid in sawing through the wafer from the top surface of the special-purpose die. The special-purpose die is one of a plurality of special-purpose dies arranged in an array on the wafer. According to one aspect of the invention, the special-purpose dies are selected from a group consisting of image sensor dies and micromachine dies.
Another embodiment of the invention includes a structure for protecting a special-purpose area located on a top surface of a special-purpose die. The structure includes a protective layer having a first polymerized upper zone and having a second lower zone. The second lower zone has at least a portion thereof unpolymerized and with an adhesive lower surface. The adhesive lower surface of the unpolymerized portion of the second lower zone is attached to the top surface of the special-purpose die so that the protective layer overlies and protects the special-purpose area of the special-purpose die. In this embodiment, the adhesive lower surface of the unpolymerized portion of the second lower zone, is attached to a top surface of a wafer comprising the special-purpose die so that a polymerized portion of the second lower zone overlies and/or contact the special-purpose area.
Also in accordance with the present invention, a method includes adhesively mounting an adhesive lower surface of a protective layer to a top surface of a die such as an image sensor die or a micromachine die. A special-purpose area on the top surface of the die is contacted and protected by the protective layer. The protective layer includes a polymerizable material, which includes the adhesive lower surface.
The method further includes rendering the adhesive lower surface of the protective layer to be nonadhesive. For example, the adhesive lower surface is rendered nonadhesive by polymerizing the polymerizable material of the protective layer with ultraviolet radiation.