US20070095786A1 - Selective reactive ion etching of wafers - Google Patents

Abstract

The invention comprises a device for assisting in the selective reactive ion etching of wafers comprising, a graphite base plate including an opening for housing a wafer, and a plurality of graphite strips that can be arranged over the graphite base plate to select a site of a wafer housed in the base plate for etching.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to selective reactive ion etching of wafers and in particular to selective reactive ion etching of wafers where the etching site can be any site on the wafer.
• 2. Description of the Prior Art
• Reactive ion etchers can be used for dry isotropic etching of dielectrics (for example, polyimides, oxides, SiO₂, and nitrides Si₃N₄). Reactive ion etchers can also be used for employing reactive ion etching for the de-processing of silicon chips.
• If selective etching of a particular site on a wafer is required there are two approaches that are currently used. In the first approach the wafer is diced by wafer cleaving or wafer sawing. The die to be etched is then extracted from the diced wafer and etched. This approach has the disadvantage of destroying the wafer and all the good dies on the wafer near the die to be etched.
• In the second approach a graphite shield with an opening the size of the die is used to cover the wafer as shown inFIG. 1. As can be seen in this Figure thetarget wafer1 is sandwiched between a chuck platen2 and a graphite shield3. The graphite shield3 includes an opening through which etching will take place. Plasma is then formed in the reactive ion etcher and isotropic reactive ion etching begins. Only the portion of the wafer directly under the opening of the graphite shield will be etched and the graphite shield prevents etching of the rest of the wafer.
• There are problems with the second approach including that only the centre die can be etched using the graphite shield. The shape and size of the hole in the graphite shield determines the size and shape of wafer that will be etched using the shield. If another shape and size wafer is to be etched another shield is needed. Further if the wafer to be etched is not in the centre of the die another shield will be needed. These expensive and heavy graphite shields require fabrication with the vendor. Using this second approach a number of different shields may be required which is costly and inefficient.
BRIEF SUMMARY OF THE INVENTION
• Accordingly in the present invention there is provided a device for assisting in the selective reactive ion etching of wafers comprising a base plate including an opening for housing a wafer and a plurality of strips that can be arranged over the base plate to select a site of a wafer housed in the base plate for etching.
• Preferably the strips and base plate are formed from graphite. Alternatively the strips and base plate may be formed from aluminium.
• Preferably the strips are all the same length.
• Preferably the strips can have different widths. If the strips have different widths, preferably the widths are multiples of a unit dimension.
• Preferably the opening in the base plate is circular.
• Preferably the site selected for reactive ion etching is opened to reactive ion etching by arranging the strips in two dimensions.
• In another embodiment there is provided a method of selectively reactive ion etching a portion of a wafer including the step of placing the wafer in a base plate, identifying the portion of the wafer to be etched, arranging a plurality of strips over the wafer and base plate to cover all portions of the wafer that are not to be etched and leave exposed the portion of the wafer to be etched and performing a reactive ion etch on the wafer.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
• The invention will be further described by way of example only and without intending to be limiting with reference to the following drawings, wherein:
• FIG. 1 is a view of a reactive ion etching process using a currently used graphite shield;
• FIG. 2 is a view of one embodiment of the invention showing the layering of graphite strips over a graphite base plate and wafer, and
• FIG. 3 is a view of a second embodiment of the invention showing the layering of graphite strips over a graphite base plate and wafer.
DETAILED DESCRIPTION OF THE INVENTION
• It should be noted the throughout the detailed description the strips are referred to as graphite strips. This is by way of example only and is not intended to limit the invention to graphite strips. The strips may be any suitable material as described below. In some embodiments the strips may be aluminium
• Referring now toFIG. 2 there is shown therein an assembly of a wafer11, graphite base plate12, and plurality of graphite strips13. For clarity some portions of the strips are shaded and others are not to show the wafer and graphite base plate beneath the strips.
• The graphite base plate provides a base on which the wafer sits. In preferred embodiment the graphite base plate includes a central aperture in which the wafer is positioned. This aperture can a circular opening be of any suitable diameter. For example the opening can be 6 inches in diameter, 8 inches in diameter, or 12 inches in diameter. In alternative embodiments the graphite base plate may include a base portion onto which the wafer is set and side portions that extend from the base portion.
• The surrounds of the graphite base plate around the central aperture or ides of the base plate extend to at least the height of the wafer and preferably beyond the height of the wafer to provide surfaces on which the graphite strips rest.
• In preferred embodiments the graphite base plate is formed from graphite and is about half an inch think between the wafer and one quarter of an inch thick around the central aperture.
• In preferred embodiments the base plate and strips are formed from graphite. In alternative embodiments the base plate and strips may be formed from aluminium. The base plate must be formed from a highly conductive material so as not to interfere with the reactive ion etching The base plate material must not be reactive to the gas species used in the reactive ion etching. These gases are mainly CHF₃, C₂F₆, CF₄, and O₂. The temperature within the reactive ion etch chamber can be as high as 100° C. so the melting point of the base plate material must be a few hundred degrees Celsius. One material that meets all of these requirements is graphite. Another suitable base plate material is aluminium. Either of these materials may be used for the base plate and strips.
• Graphite strips13 are positioned on the graphite base plate after a wafer is placed on the graphite base plate. In the preferred embodiment graphite strips13 are all the same length. In this embodiment the graphite strips are laid out parallel to the x axis and y axis as shown inFIG. 2. The graphite strips may have different widths. In this embodiment it is preferred that the widths of the graphite strips are multiples of a unit dimension. For example the unit dimension may be the typical width of a die within the wafer. InFIG. 2 strips13aand13bare twice the width of the remaining graphite strips. In other embodiments some graphite strips may be 3 or more times as wide as the least wide graphite strip.
• In one embodiment in a cross-sectional or side view the strips form a straightened Z shape with parallel upper and lower members and a connecting member between the upper and lower members. In preferred embodiments the connecting member is an overlap between the upper and lower members. In alternative embodiments the connecting member may be a separate member and is preferably at right angles to the upper and lower members. This arrangement of the strips allows the strips to overlap when arranged around a target site. The overlapping arrangement assists in protecting the wafer under the strips from the etching process.
• In use when a wafer is to be selectively etched the wafer is first placed on the graphite base plate. If the graphite base plate includes a circular aperture the wafer should be placed in the aperture. After this the portion of the wafer to be etched is identified. If the wafer if circular and placed in a circular aperture in the graphite base plate preferably the wafer is aligned within the base plate so that the sides of the portion of the wafer to be etched are parallel to the sides of the base plate (as shown inFIG. 2). The graphite strips are then placed over the wafer and base plate to cover all areas of the wafer that are not to be etched and leave exposed the portion of the wafer to be etched. InFIG. 2 this is shown by the different shading on the strips. The shaded areas show the intersection every second x-axis strip with every second y-axis strip. The clear areas are covered by strips by have been left clear to show the wafer and graphite base plate below the graphite strips. Onlyportion14 is left exposed. A reactive ion etch is then performed. This etch will only affect the exposed portion of the wafer.
• The graphite strips are preferably laid in one direction and then the other. For example, the wafer strips may be laid parallel to the x-axis and then a second layer may be laid parallel to the y-axis.
• In alternative embodiments the axes chosen for laying the graphite strips may be different. In these alternative embodiments the lengths of the graphite strips may be different. It should also be noted that the axes along which the strips are laid are orthogonal in the preferred embodiment but need not be so long as the wafer is covered leaving only the selected portion to be etched.
• FIG. 3 shows another embodiment of the invention. In this embodiment there are five strips16a-16ethat run parallel to the x-axis and eight strips17a-17hthat run parallel to the y-axis to exposetarget site15.
• As can be seen inFIG. 3, the width of the strips parallel to the y-axis range from unit dimension width, strip17a, to three times the unit dimension width, strips17e, f,andh.The width of the strips parallel to the x-axis range from twice the unit dimension width,16a, to seven times the unit dimension width,16e. The strips along one axis are laid first omitting a strip that win cover the target site. Then the strips along the other axis are laid, again omitting a strip that will cover the target site. This means that all areas of the wafer are covered by two strips except the areas in line with the target site on the x and y axes and the target site. It should be noted that in this embodiment the size of the target site is not the same as the minimum width of the graphite strips.
• The advantages of the present invention include that it provides the flexibility to perform a reactive ion etch on any selected site or location on the target wafer. The present invention also allows target sites of differing sizes, in fact the target site can be any size required. The invention also protects the unexposed areas of the wafer from the reactive ion etch to allow further testing. Another advantage is that the invention is simple to implement and is highly effective.
• During reactive ion etching chemical etching and physical ion sputtering occurs. Some of the etch species or compounds can be redeposited back onto the surface of the wafer. Use of the strips of the invention prevents re-deposition of etched species or compounds onto the non-selected areas.
• The foregoing describes the invention including preferred forms thereof Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated in the scope hereof as defined by the accompanying claims.

Claims (2)

1.-11. (canceled)

12. A method of selectively reactive ion etching a portion of a wafer comprising the steps of,

placing the wafer in a base plate,

identifying the portion of the wafer to be etched,

arranging a plurality of strips over the wafer and base plate to cover all portions of the wafer that are not to be etched and leave exposed the portion of the wafer to be etched, and

performing a reactive ion etch on the wafer.

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