Abstract:
The present description describes back-end processes, the use of which may help overcome these problems and limitations of the prior art. In one optional embodiment, the back-end process includes depositing a layer over a wafer. The wafer contains a plurality of circuit die for respective RFID tags. The wafer also has exposed metallic regions. The exposed metallic regions include first regions having electrical contacts to the plurality of circuit die and second regions having electrical contacts to the wafer&#39;s electrical test sites. The method includes forming exposed first regions and unexposed second regions by etching the layer over the first regions but not over the second regions. The method also includes plating metallic bumps on the exposed first regions.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present description is related to the field of electric circuitry, and more specifically to an RFID tag circuit die with an additional file to control I/O bump flow. 
   2. Description of the Related Art 
     FIGS. 1A ,  1 B and  1 C are snapshots of steps in methods for preparing integrated circuit chips from a silicon wafer according to embodiments. 
   Integrated circuits are made according to embodiments, using semiconductor fabrication methods. A quick overview of these methods is now described. 
     FIG. 1A  shows a starting wafer  191 . Wafer  191  is typically of semiconductor material, such as silicon. The silicon is sometimes doped with p-type or n-type impurities, to improve its electronic properties as desired. Wafer  191  has an original surface  192 , at which circuits are formed as described below. 
     FIG. 1B  shows a wafer  194 , which is derived from wafer  192  after circuits  195  have been formed according to embodiments. Circuits  195  are formed by semiconductor manufacturing machines, often operated by foundries. It is worthwhile to note that circuits  194  are formed at surface  192 , both beneath its original level and above it. Accordingly, wafer  194  has a new surface, which is elevated compared to original surface  192 . 
     FIG. 1C  shows that wafer  194  of  FIG. 1B  is afterwards separated into chips  197 ,  198 ,  199 . Separation can happen by dicing wafer  194 , or etching it, etc. Each of chips  197 ,  198 ,  199  typically contains one of circuits  195 , and is thus called an integrated circuit (IC) chip or “circuit die”. The size of each IC chip is thus determined in part by the size of circuit  195 . 
     FIGS. 2A through 2E  demonstrate a particular problem with the state of the art of manufacturing circuit die for use in RFID tags.  FIG. 2A  shows a cross section of a completed wafer before it has been diced into multiple individual circuit die.  FIG. 2A  shows the wafer substrate  201  and the overlying layers  205  of wiring metallurgy and dielectric insulation. At the top of the processed wafer are bump pads  202   a,b  and test pad  203 . 
   Bump pads  202   a,b  are “I/O” pads meaning they are used to support later manufactured “bumps” or “contacts” that supply electrical signals to and/or from the semiconductor die of which they are a part. As will be seen, pad  202   a  and pad  202   b  belong to two separate die (that is, pad  202   a  belongs to a first die and pad  202   b  belongs to second die). As will be observed further below, the bumps that are later formed on the I/O pads on an RFID tag circuit die are typically connected to an antenna assembly after the die has been separated from the wafer. 
   Also observed on the surface of the wafer is a test pad  203 . The test pad is a form of I/O used by the wafer processing manufacturer (e.g., a foundry) to test the quality of the structures within wiring layer  205  and/or the quality of the transistors that are embedded within the substrate  201 . Here, test structures, called test sites, are purposely created within the wiring layer  205  and the substrate  201  and pad  203  (among other pads like pad  203 ) are electrically to respective one or more test site(s). As will be seen, the wafer is cut into individual die by sawing through a region of the wafer at or near to the test pad  203 . 
   The depiction of  FIG. 2A  shows the wafer after the wafer is near completion. Here, note that the pads  202   a ,  202   b  and  203  can be viewed as being “exposed” through a layer of passivation  204   a ,  204   b ,  204   c ,  204   d . During the final stages of wafer processing, referred to as “back-end” processing, after the pads  202   a ,  202   b ,  203  are formed, a layer of passivation is coated over the wafer thereby covering the pads  202   a ,  202   b ,  203 . The passivation layer is subsequently patterned such that openings in the passivation layer are formed directly above the pads  202   a ,  202   b ,  203  thereby exposing them. 
     FIG. 2B  shows the beginning of the process of separating the wafer into individual die. According to the specific depiction of  FIG. 2B , the wafer is sawed along a saw street  211  which, according to the particular depiction observed, is along the wafer test pad  203 . The particular depiction observed also indicates that the wafer is not completely sawed through. 
     FIG. 2C  next shows the formation of the aforementioned I/O bumps  206   a ,  206   b . According to one process, the bumps  206   a ,  206   b  are made substantially of Nickel (Ni) and Gold (Au), e.g., Ni x Au x-1 , and are deposited by a plating process such as electroplating. According to other processes the bumps  206   a ,  206   b  may be made substantially of Au or Palladium (Pd) or solder. In a plating processing, the bumps are deposited on their respective bump pads by activating one or more chemical reactions between the exposed pads  202   a ,  202   b  and a liquid solution that is applied to the surface of the wafer. Depositing bumps in this fashion is well known in the art. 
   The liquid solution used to form the bumps, however, substantially wets the entire surface of the wafer causing chemical reaction at regions of the wafer other than the exposed pads  202   a ,  202   b . Because the solution is designed to react with the metal of the bump pads  206   a ,  206   b  to form the bumps, other exposed metallic regions are apt to promote the deposition of bump material. Because the metallurgy associated with the wafer test pad  203  and the wiring layer  205  of the wafer may include metal that is the same as or similar to the metal of the bumps pads  202   a ,  202   b , these regions tend to promote the formation of unwanted bump material  206   c ,  206   d . Note that if the wafer is not sawed at all the test pad  203  by itself can support the formation of unwanted bump material. 
     FIG. 2D  shows the next step in the process which entails the separation of the wafer into individual die. This can be done either by continuing the sawing or by “back-grinding” the back of the wafer until the surface of back of the wafer meets the end of the sawed to depth. 
     FIG. 2E  shows a problem that is particularly acute with respect to RFID circuit die. According to the depiction of  FIG. 2E , when the antenna assembly is attached to the circuit die, the antenna inlay  208   a  makes electrical contact (“shorts”  266 ) not only with the bump  206   a  (where electrical contact is supposed to be made) but also with the unwanted bump material. The antenna receives high frequency signals. Electrical channels that process the received high frequency signals are usually designed with an “impedance” that is precisely attuned to diminish the ill-effects of signal attenuation and/or reflections along the channel. 
   The impedance of an electrical channel is typically affected by the capacitances, inductances and/or resistances associated with the channel. Here, the contact between the inlay  208   a  and the unwanted bump material adversely affects these features of the channel such that the impedance of the channel deviates from its designed-for value. The result is a marginally operable or inoperable RFID tag. 
   BRIEF SUMMARY 
   The present description describes back-end processes, the use of which may help overcome these problems and limitations of the prior art. In one optional embodiment, the back-end process includes depositing a layer over a wafer. The wafer contains a plurality of circuit die for respective RFID tags. The wafer also has exposed metallic regions. The exposed metallic regions include first regions having electrical contacts to the plurality of circuit die and second regions having electrical contacts to the wafer&#39;s electrical test sites. The method includes forming exposed first regions and unexposed second regions by etching the layer over the first regions but not over the second regions. The method also includes plating metallic bumps on the exposed first regions. 
   Advantages over the prior art may include improving RFID tag yield through the elimination of unwanted contact bump metal and associated effects; reduction of photosensitivity RFID tag circuit die; reduction of thermal stress applied to the die during while the die is being packaged; reduction of parasitic capacitance through dielectric constant reduction; enhanced adhesion between antenna and die; improved packaging yield through die stress buffering relief. 
   These and other features and advantages of this description will become more readily apparent from the following Detailed Description, which proceeds with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
       FIG. 1  shows how RFID tag integrated circuits can be made from a silicon wafer according to embodiments; 
       FIG. 2A  (prior art) shows a plurality of RFID tag circuit die formed on a wafer such as the wafer of  FIG. 1 , whose bump pads are exposed; 
       FIG. 2B  (prior art) shows the wafer of  FIG. 2A  after it has been partially sawed according to a conventional process; 
       FIG. 2C  (prior art) shows the wafer of  FIG. 2B  after Ni x Au x-1  bumps have been formed on the exposed bump pads according to a conventional process; 
       FIG. 2D  (prior art) shows separate circuit die resulting after backgrinding the wafer of  FIG. 2C  according to a conventional process; 
       FIG. 2E  (prior art) shows the circuit die of  FIG. 2D  after antenna inlays have been attached to their respective bumps according to a conventional process; 
       FIG. 3  shows flow charts for describing improved methodologies of forming RFID tag circuit I/O bumps according to embodiments; 
       FIG. 4A  shows a plurality of RFID tag circuit die formed on wafer such as the wafer of  FIG. 1 , whose bump pads are exposed; 
       FIG. 4B  shows the wafer of  FIG. 4A  after a layer of polyimide ( 410 ) has been formed on the wafer; 
       FIG. 4C  shows the wafer of  FIG. 4B  after the polyimide layer has been etched above the bump pads ( 402   a,b ), but not over the wafer test cite pads ( 403 ); 
       FIG. 4D  shows the wafer of  FIG. 4C  after Ni x Au x-1  bumps have been formed on the exposed bump pads; 
       FIG. 4E  shows the wafer of  FIG. 4D  after it has been partially sawed; 
       FIG. 4F  shows the wafer of  FIG. 4E  after it has been back grinded to form separate circuit die; and 
       FIG. 4G  shows the circuit die of  FIG. 4F  after antenna inlays have been attached to their respective bumps. 
   

   DETAILED DESCRIPTION 
     FIGS. 3 ,  4 A through  4 G and the following discussion demonstrate an improvement over the prior art.  FIG. 4A , like  FIG. 2A , shows the wafer having a layer of passivation  404   a ,  404   b ,  404   c ,  404   d  through which bump pads  402   a ,  402   b  and test pad  403  are exposed. However, instead of immediately beginning to separate the wafer into die (as seen in  FIG. 2B ), another layer  410  is deposited  301  over the wafer as seen in  FIG. 4B . This layer is referred to as a shielding layer because, as will be described below, it shields the wafer test pads from the bump deposition solution thereby preventing the formation of unwanted bump material on the wafer surface. 
   According to one embodiment, the shielding layer  410  is made substantially of polyimide. According to another embodiment, the shielding layer  410  is a second layer of passivation. Processing details concerning these layers are provided in more detail below. After shielding layer  410  is deposited over the wafer, it is patterned  302  so as to expose the bump pads  402   a ,  402   b  but not the test pad  403 . The resulting structure is observed in  FIG. 4C . 
   The patterning may be performed by traditional lithography techniques where a layer of photo-resist is coated over shielding layer  410  and then electromagnetic radiation is directed through a mask onto the photoresist. 
   The patterns on the mask are such that the radiation shines on the regions of the photoresist over the bump pads  402   a ,  402   b  but does not shine over the regions of the photoresist over the test pads  403  (if the photoresist is of a first polarity), or, the patterns on the mask are such that the radiation shines on the regions of the photoresist over the test pads  403  but does not shine over the regions of the photoresist over the bump pads  402   a ,  402   b  (if the photoresist is of a second polarity). Alternatively, certain types of polyimides can be exposed directly with radiation (i.e., without a layer photoresist). 
   Importantly, wafer level testing where test signals are applied and/or received through test pads such as test pad  403  are performed before shielding layer  410  is deposited on the wafer. So, for example, a standard processing flow might entail the following sequence: 1) process the wafer up to the structure of  FIG. 4A ; 2) perform wafer level testing by applying wafer test probes to test pads such as test pad  403 ; 3) deposit shielding layer  410  to form the structure of  FIG. 4B  on a tested wafer; 4) pattern shielding layer  410  to form the structure of  FIG. 4C . 
   Starting then with the structure of  FIG. 4C , contact bumps  206   a ,  206   b  are deposited  303  on the exposed bump pads  402   a ,  402   b , for example, by any of a number of plating processes as described above in the Background. Importantly, because the test pad  403  is not exposed, it cannot act as a catalyst for depositing unwanted bump material as described in the Background. Thus, the result of the plating process is properly formed bump material substantially limited to the desired bumps residing on the bump pads. 
   After the bumps  206   a ,  206   b  are formed, the wafer is separated into individual die. Thus, as observed in  FIG. 4E , the wafer is cut along the saw street  411 . Here, polyimide and/or passivation is a somewhat “hard” material. Hence, according to one perspective, in order to minimize wafer saw blade wear-out, the thickness of shielding layer  410  should be kept minimal (e.g., approximately within a range of 2-6 μm inclusive). However, as described in more detail below, certain ancillary or additional advantages besides elimination of unwanted bump material may result. Because some of these advantages are better realized with a greater thickness for shielding layer  410 , the ultimate thickness for shielding layer  410  depends on weighing the various advantages described below. 
     FIG. 4F  shows the separation of the wafer into individual die through back grinding. Again, alternate separation techniques can be used (such as sawing straight through the wafer).  FIG. 4G  shows the antenna inlay  408   a  attached to the separated die. Note that, unlike the depiction of  FIG. 2E , unwanted bump material is not in contact with the antenna inlay  408   a . A discussion of some possible features of shielding layer  410  follows immediately below. 
   As discussed above, shielding layer  410  can be formed at least with polyimide or a second layer of passivation. A shielding layer  410  made of polyimide may be easily applied via standard spin-on or deposition techniques. The polyimide layer may be non-soluble, non-conductive as well as resistant to mechanical and thermal stresses. An exemplary polyimide includes Kapton® made by DuPont®. Shielding layer  410  can also be implemented as a second passivation shielding layer  410 . Passivation is typically implemented as Si 3 N 4  or other materials such as silicon oxynitride or doped glasses. Immediately following is a discussion of some additional or ancillary advantages of the shielding layer  410  that may affect some of the characteristics and/or processing parameters discussed just above. 
   One such ancillary advantage is the reduction of light sensitivity effects. RFID tag circuit die are essentially weak signal devices. That is, the electrical signal produced by the antenna is not a particularly strong signal. Silicon exhibits some photo-sensitivity, thus, electrical currents are produced in a RFID tag circuit die when light is shined on the die. Because of the relatively weak signals that an RFID tag circuit die typically processes, the presence of light can interfere with the electrical signaling and therefore interfere with the proper operation of the RFID tag. 
   Within the visible and Ultra Violet (UV) spectra at wavelength below 550 nm, a layer of polyimide tends to absorb light rather than transparently permit the light to flow through it. The absorption, in turn, reduces the intensity of light that reaches the semiconductor wafer. Thus, shielding layer  410  can be used to diminish the photosensitivity issues associated with RFID tag circuit die at least at wavelengths below 550 nm. Conceivably the polyimide could be darkened (e.g., painted black) to enhance the spectrum over which it exhibits absorptive behavior. 
   Another possible advantage is the role of shielding layer  410  as a mechanical stress buffer. Semiconductor ICs, when packaged, are subjected to a process in which epoxy resin is heated and molded around the IC. The thermal expansion of the resin places mechanical stress on the die which can result in its cracking or warping. Therefore, using a shielding layer that is pliable or compressible should absorb some of the stress that would otherwise be applied to the die during packaging. Also, when the antenna inlay is attached to the die, an epoxy resin is used that must be cured with high temperatures. Again, a layer of polyimide can protect the die against any resulting mechanical stresses by absorbing the expansion of the epoxy. Furthermore, polyimide can also promote adhesion to resin thus resulting in better adhesion between the die and the antenna inlay. 
   Also, additional non-thermally induced mechanical stresses can be applied to a die during its packaging. Shielding layer  410  can protect against chipping or cracking of the die resulting from the application of such mechanical stresses. For instance, according to various packaging processes, individual die are attached to backing tape on their wafer substrate side before they are packaged into individual packages. In order to remove the die from the backing tape a blunt pin is forcibly protruded through the backing tape upon the wafer substrate side of the die (e.g., to “pop” the die off the backing tape). The force of this trauma can cause chipping or cracking of the die. However, a polyimide shielding layer  410  can act as a sealant or other protective coating that prevents the die from being damaged in this manner. Conceivably, if the shielding layer  410  is hard enough, the tape can be applied to the surface of the die rather than the backside because the hard shielding layer can withstand the blunt pin force applied to the metallization surface of the die. 
   If the die is placed with the top of the die (surface with polyimide) on the tape, then the polyimide may also act as a soft buffer to absorb some of the trauma associated with removing the die from the tape. An example of a process that results in the die having its front attached to the tape is as follows. One possible process is for the saw operation to only partially go through the topside of the wafer. Then the wafer is transferred to another frame with tape face down (die top attached to the tape). The wafer is then back ground. According to a back grinding process, the exposed bottom of the die/substrate is ground or polished (material removed). When the back grind operation removes enough material to reach the bottom of the saw street, the die are singulated. This process results in separated die on tape with the “top” side attached to the tape. This process might be advantageous to manufacturing since the machine that attaches to the die during the removal from tape process can then immediately place the die on a tag with the bumps down (as needed to make electrical contact to the antenna. 
   Another possible benefit is the reduction of parasitic capacitance associated with the antenna assembly including the antenna inlay. Essentially, a polyimide layer having a dielectric constant less than 3.9 (e.g., within a range of 3.0 to 3.8 inclusive) can be readily deposited. Without the presence of shielding layer  410 , the volume consumed by shielding layer  410  will include passivation or other material having a higher dielectric constant (e.g., SiO2 at 3.9 or Si3N4 at 7.5). Capacitance is proportional to dielectric constant, thus, lower dielectric constant materials between the antenna inlay and the die should result in lower parasitic capacitances between the inlay and die. These in turn can provide greater tolerance of inlay/bump misalignment and tighter impedance tolerances. 
   In the above, the order of operations is not constrained to what is shown, and different orders may be possible. In addition, actions within each operation can be modified, deleted, or new ones added without departing from the scope and spirit of the invention. 
   The electrical circuit(s) described in this document can be manufactured in any number of ways, as will be appreciated by the persons skilled in the art. One such way is as integrated circuit(s), as described below. 
   Schematic-type inputs can be provided for the purpose of preparing one or more layouts. These inputs can include as little as a schematic of a circuit, to more including relative sizes of circuit components and the like, as will be appreciated by a person skilled in the art for such inputs. These inputs can be provided in any suitable way, such as merely in writing, or electronically, as computer files and the like. Some of these computer files can be prepared with the assistance of suitable design tools. Such tools often include instrumentalities for simulating circuit behaviors and the like. 
   These inputs can be provided to a person skilled in the art of preparing layouts. This, whether the person is within the same company, or another company, such as under a contract. 
   A layout can be prepared that embodies the schematic-type inputs by the person skilled in the art. The layout is itself preferably prepared as a computer file. It may be additionally checked for errors, modified as needed, and so on. 
   In the above, computer files can be made from portions of computer files. For example, suitable individual designs can be assembled for the electrical components and circuits indicated in the schematic-type inputs. The individual designs can be generated anew, or selected from existing libraries. In the layout phase, the assembled designs can be arranged to interoperate, so as to implement as integrated circuit(s) the electrical circuit(s) of the provided schematic-type inputs. These computer files can be stored in storage media, such as memories, whether portable or not, and the like. 
   Then a special type of computer file can be synthesized from the prepared layout, in a manner that incorporates the prepared layout, which has the embodied schematic-type inputs. Such files are known in the industry as IC chip design files or tapeout files, and express instructions for machinery as to how to process a semiconductor wafer, so as to generate an integrated circuit that is arranged as in the incorporated layout. 
   The synthesized tapeout file is then transferred to a semiconductor manufacturing plant, which is also known as a foundry, and so on. Transferring can be by any suitable means, such as over an electronic network. Or a tapeout file can be recorded in a storage medium, which in turn is physically shipped to the mask manufacturer. 
   The received tapeout file is then used by mask making machinery as instructions for processing a semiconductor wafer. The wafer, as thus processed, now has one or more integrated circuits, each made according to the layout incorporated in the tapeout file. If more than one, then the wafer can be diced to separate them, and so on. 
   In this description, numerous details have been set forth in order to provide a thorough understanding. In other instances, well-known features have not been described in detail in order to not obscure unnecessarily the description. 
   A person skilled in the art will be able to practice the present invention in view of this description, which is to be taken as a whole. The specific embodiments as disclosed and illustrated herein are not to be considered in a limiting sense. Indeed, it should be readily apparent to those skilled in the art that what is described herein may be modified in numerous ways. Such ways can include equivalents to what is described herein. 
   The following claims define certain combinations and subcombinations of elements, features, steps, and/or functions, which are regarded as novel and non-obvious. Additional claims for other combinations and subcombinations may be presented in this or a related document.