METHOD AND SYSTEM FOR REDUCING PHOTO-ASSISTED CORROSION IN WAFERS DURING CLEANING PROCESSES

A method and system are provided for reducing and eliminating photo-assisted copper corrosion during a wafer cleaning process. In one example, a method is provided for making a composite material for the cover of a wafer cleaning system. A first transparent layer is formed. A transparency tunable layer is formed over the first transparent layer. Electrical connections are defined between a first portion and a second portion of the transparency tunable layer. And a second transparent layer is formed over the transparency tunable layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An invention for methods and systems for reducing photo-assisted copper corrosion during a wafer cleaning process are disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. FIGS. 3A, 3B , and 3 C show a top view and side views, respectively, of a wafer cleaning system, in accordance with one embodiment of the present invention. The wafer cleaning system 300 typically includes an input station 302 where a plurality of wafers may be inserted for cleaning through the system after the wafers have undergone CMP operations. Once the wafers are inserted into the input station 302 , a wafer may be taken from the input station 302 and moved into the brush box 304 , which contains a first brush box 304 a and a second brush box 304 b. Inside the brush box, various cleaning operations may be applied to the wafer. After brushes have been applied to the wafer in the brush boxes 304 , the wafer is moved into a spin, rinse, and dry (SRD) station 306 . In the SRD station 306 , de-ionized (DI) water is sprayed onto the surface of the wafer while the wafer is spun at a speed of between about 100 and 400 revolutions per minute, and then is spun to dry. After the wafer has been placed through the SRD station 306 , an unload handler 308 takes the wafer and moves it into an output station 310 . The cleaning system 300 is programmed and controlled from system electronics 312 . The transparency level of the cover of the wafer cleaning system 300 is preferably tunable from being substantially transparent to being opaque, as shown in FIG. 3C . The “cover” is the portion of the wafer cleaning system that houses the wafer cleaning operations. The term “substantially transparent” means substantially all of the light that is directed toward the outer surface of the cover passes through the cover. The term “opaque” means about none of the light that is directed toward the outer surface of the cover passes through the cover. The “outer surface of the cover” refers to the surface of the cover that is not facing the wafer cleaning operations. The term “light” refers to light that is within the ultra-violet (UV) and visible spectrum. Depending on the material used to construct the cover, a change in transparency may be accompanied by a corresponding change in color, as further discussed below with reference to FIG. 4A . When the cover is substantially transparent, a user can view the cleaning process. However, as discussed in greater detail above, light energy may assist in corroding copper lines when cleaning is performed after a copper CMP process. Accordingly, the present invention provides a cover for the cleaning system that preferably can be tuned to be opaque when cleaning operations are being performed and substantially transparent when the cleaning is not being performed. In certain cases, it may be desired to run a cleaning operation when the cover is substantially opaque, but the inner cleaning operations can still be viewed. This will allow an operator to determine whether the brushes are operating properly, and the like. FIGS. 4A and 4B show a side view and a top view, respectively, of a composite material 400 used for the cover on a wafer cleaning system, in accordance with one embodiment of the present invention. The composite material 400 preferably includes a first transparent layer 404 a, a second transparency layer 404 b, and a transparency tunable layer 406 coated between the transparency layers 404 . The transparency layers 404 are preferably a clear acrylic material. Although, other known plastics and/or glass can also be used. The transparency tunable layer 406 is preferably a photochromic or electrochromic material, such as tungsten oxide (WO 3 , WO x ). Alternative materials can include, for example, NB 2 O 5 , V 2 O 7 , TiO 2 , ZnO, Cr 2 O 3 , MnO 2 , CoO, NiO 2 . Any one of these materials can also be implemented depending on the specific application. For purposes of this exemplary discussion, reference will be made to tungsten oxide. To create the composite material, the transparency tunable layer 406 is preferably sputtered onto the first transparency layer 404 a (or the second transparency layer 404 b ). Another technique is a spin-on technique, where the transparency tunable layer 404 a is, for example, formed by a “sol-gel” process. The second transparency layer 404 b is formed atop the transparency tunable layer 406 . Sets of electrical connections 402 a and 402 b are conductively integrated to portions of the transparency tunable layer 406 . When a bias voltage V &plus; is applied across the transparency tunable layer 406 between the portions, an electrical circuit defined by the electrical connections 402 and the transparency tunable layer 406 is closed. As shown in a preferred embodiment in FIG. 4, a first portion is on a first side of the transparency tunable layer 406 , and a second portion is on a second side of the transparency tunable layer 406 . As the desired voltage application V &plus; is increased, a current I that runs across the transparency tunable layer 406 proportionately increases. This increase in current causes electrons e − to flow and excite the atoms in the photochromic or electrochromic material. This excitation of atoms causes a change in transparency level, which may be accompanied by a change in color. Tungsten oxide, for example, is a light yellowish color in a lesser excited state, thereby making the tunable layer 406 substantially transparent. Tungsten oxide is a dark blue color in a more excited state, thereby making the tunable layer 406 opaque. In sum, a low voltage V &plus; causes the cover to be substantially transparent, while a high voltage V &plus; causes the cover to be opaque. Generally, the voltage V &plus; preferably ranges from between about 0.5 volts and about 3 volts, more preferably between about 1 volt and about 1.5 volts, and most preferably about 1.25 volts. Where tungsten oxide (WO 3 ) is used, the voltage V &plus; preferably ranges from between about 0.5 volts and about 5 volts, and most preferably about 3 volts. The dimensions of the composite material 400 are preferably defined by at least two parameters, the cover thickness b and the tunable layer thickness a. The cover thickness b is preferably about 1 cm. The tunable layer thickness a is preferably between about 0.5 &mgr;m and about 10 &mgr;m, and most preferably about 3 &mgr;m. FIG. 5 shows a high-level schematic diagram of preferred system components for the tunable transparency cover, in accordance with one embodiment of the present invention. A voltage controller 502 has electrodes (not shown) coupled to the electrical connections 402 and, thereby, establishes a bias voltage V &plus; across the transparency tunable layer 406 . Tuning control circuitry 504 that receives input from a control unit 506 provides the appropriate state for the voltage controller 502 . The control unit 506 provides a user with operation control 512 and emergency control 514 . When the user is using the operation control 512 , the tuning control circuitry 504 provides a state of regular operation 510 to the voltage controller 502 . Operation control 512 allows the user to tune the voltage low or high, depending on the transparency level that is required. When the user is using the emergency control 514 , the tuning control circuitry 504 preferably provides a voltage shut-off to the voltage controller 502 . When the voltage is shut-off, the composite material 400 is preferably in about its most transparent state. The emergency control 514 may be desired for cases when the cleaning system experiences a problem, e.g., a broken wafer, and the user needs to ascertain the problem immediately. In other cases, the emergency control 514 will be advantageous when the power unexpectedly shuts off and the operator needs to view the inside of the cleaner to determine the current state of a cleaning session. FIG. 6A shows a flow chart of a method for forming a composite material 400 , in accordance with one embodiment of the present invention. The method starts in operation 702 where a first transparent layer is formed. The method then proceeds to operation 704 where a transparency tunable layer is formed over the first transparent layer. The transparency tunable layer preferably has characteristics such as those discussed with reference to FIGS. 4A and 4B . Next, the method moves to operation 706 where electrical connections are defined between a first portion and a second portion of the transparency tunable layer. The method then moves to operation 708 where a second transparent layer is formed over the transparency tunable layer. FIG. 6B shows a flow chart of a method for forming a transparency tunable cover for a wafer cleaning system, in accordance with one embodiment of the present invention. The method starts in operation 802 where a first transparent layer is formed for a semiconductor cleaning station cover. The method then proceeds to operation 804 where a transparency tunable layer is formed over the first transparent layer. The cover preferably has electrodes at appropriate ends to enable circuitry to couple thereto and enable a current flow, as discussed with reference to FIG. 4B . The current flow through the cover will therefore enable the cover to change in transparency. When the cleaning system in operational, and the cleaning is being performed after a copper CMP, the photo-assisted corrosion will be advantageously prevented. This is a substantial advance in cleaning technology, in that conventional cleaning systems all use one-state clear covers that allow light to freely pass therethrough. A cleaning system using this tunable cover can now program the state of transparency to be substantially dark when the cleaning is in progress and light when no cleaning operation is being performed. Of course, the level of transparency can vary anywhere in between each extreme, depending on the users needs and the type of cleaning being performed. Next, the method moves to operation 806 where electrical connections are defined between a first portion and a second portion of the transparency tunable layer. The method then moves to operation 808 where a second transparent layer is formed over the transparency tunable layer. While this invention has been described in terms of several preferred embodiments, it will be appreciated that those skilled in the art upon reading the preceding specifications and studying the drawings will realize various alterations, additions, permutations and equivalents thereof. For example, although specific reference is made to brush boxes, any other brush scrubbing apparatus can benefit from the method teachings of the present invention. Additionally, the cleaning embodiments can be applied to any size wafer, such as, 200 mm, 300 mm, and larger, as well as other sizes and shapes. It is therefore intended that the present invention includes all such alterations, additions, permutations, and equivalents that fall within the true spirit and scope of the invention.