Abstract:
A method of forming a plurality of micro column interconnection structures on a semiconductor includes providing a semiconductor layer. A photoresist layer is formed on the semiconductor layer. A plurality of cavities are etched in the photoresist layer. The plurality of cavities extend through the photoresist layer to the semiconductor layer. Solder is deposited in the plurality of cavities, thereby forming a plurality of micro columns of solder.

Description:
THE FIELD OF THE INVENTION 
     This invention relates generally to methods for forming interconnection structures for electronic chips. This invention relates more particularly to a method for forming a micro column grid array (CGA). 
     BACKGROUND OF THE INVENTION 
     Two levels of interconnection structures are typically used to join a semiconductor die to a printed circuit board. A first level of interconnection is typically provided between a semiconductor die and a substrate, and then a second level of interconnection is provided between the substrate and the printed circuit board. 
     One common technique of packaging a semiconductor die is to flip-chip join a semiconductor die to a ceramic substrate. The flip chip approach offers significant advantages in terms of input/output (I/O), packaging density, electrical performance, and manufacturability of a module. A traditional method for achieving a flip chip interconnect is to deposit solder bumps onto an active surface of a semiconductor die. The solder bumps ultimately get reflowed into a spherical shape when making the connection between the semiconductor die and contact pads on a substrate. Electrical traces extend through the substrate to an opposing surface thereof, and connect to an array of solder pads, which are typically spaced apart from each other on a much larger scale than the bumps on the semiconductor die. While this approach has been proven effective, it does have limitations in the achievable I/O contact density, and more significantly, in the reliability of the connections. The reliability issue is due to the fact that a silicon chip and the next level of packaging (e.g., ceramic or organic packaging) experience very different rates of thermal expansion and contraction, and solder has particularly poor fatigue properties. 
     The problem of differing rates of thermal expansion has been addressed at the board level by attaching components to organic circuit boards with metal columns, which is a form of surface mount technology. Surface mount technology has gained acceptance as a preferred method for joining electronic components to printed circuit boards. Conventional surface mount technology approaches include ball grid arrays (BGAs) and column grid arrays (CGAs). A BGA is an array of spherical solder balls that are attached to a chip, and are used to mount the chip to a printed circuit board. A CGA is an array of cylindrical solder columns that are attached to a chip, and are used to mount the chip to a printed circuit board. The columns offer the same density and performance as a spherical attachment element, but the taller configuration of the solder columns offers compliancy to better absorb the differential thermal expansion rate between the component and the board. The compliancy virtually eliminates the reliability concern caused by the thermal expansion mismatch. 
     At the wafer level, a technique has been proposed for forming solder columns using solder jetting technology. Such a technique is disclosed in U.S. Pat. No. 6,114,187, issued Sep. 5, 2000, and entitled “METHOD FOR PREPARING A CHIP SCALE PACKAGE AND PRODUCT PRODUCED BY THE METHOD”. In the &#39;187 patent, printing technologies are used to print a package directly onto the semiconductor wafer. The &#39;187 patent notes that the prior art does not print the package directly onto a semiconductor wafer, and that it would be advantageous to build the package directly onto the semiconductor wafer and then divide the wafer into individual semiconductors. The technique disclosed in the &#39;187 patent involves forming solder columns with a solder jetting device that drops successive droplets of solder on top of each other, until the desired column height is reached. 
     There are several limitations in using solder jetting techniques, such as those disclosed in the &#39;187 patent. First, there are processing speed limitations, in that only one solder column can be formed at a time, unless multiple solder jetting devices are used, which adds to the complexity of the system. Second, it appears that only one wafer can be processed at a time. Third, the repeatability of the process for large-scale fabrication is questionable. The techniques appear to be more appropriate for small, custom applications. 
     It would be desirable to provide a unique method of forming solder columns using wafer-level processing, without the complexity or limitations of solder jetting technology. 
     SUMMARY OF THE INVENTION 
     One form of the present invention provides a method of forming a plurality of micro column interconnection structures on a semiconductor. A semiconductor layer is provided. A photoresist layer is formed on the semiconductor layer. A plurality of cavities are etched in the photoresist layer. The plurality of cavities extend through the photoresist layer to the semiconductor layer. Solder is deposited in the plurality of cavities, thereby forming a plurality of micro columns of solder. 
     Another form of the present invention provides a method of preparing a chip scale package. A microelectronic device having a connection surface is provided. A photoresist layer is formed on the connection surface of the microelectronic device. A plurality of cavities are formed in the photoresist layer. The plurality of cavities extend through the photoresist layer to the connection surface. Solder is deposited in the plurality of cavities, thereby forming a plurality of solder columns on the connection surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1D are cross-sectional diagrams illustrating one embodiment of a process for forming a micro column grid array according to the present invention. 
     FIGS. 2A-2C are cross-sectional diagrams illustrating one embodiment of a process for extending the height of the columns in a micro column grid array according to the present invention. 
     FIGS. 3A-3E are cross-sectional diagrams illustrating an alternative embodiment of a process for forming a micro column grid array according to the present invention, including the formation of a protective layer that surrounds the micro columns. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     FIGS. 1A-1D are cross-sectional diagrams illustrating one embodiment of a process for forming a micro column grid array according to the present invention. As shown in FIG. 1A, a layer of photoresist  100  is formed on a connection surface or active layer  101  of a semiconductor layer  102 . Photoresist layer  100  is then etched to provide openings or cavities  103  where solder columns will be formed. In one form of the invention, cavities  103  are generally cylindrically shaped. In one embodiment, semiconductor layer  102  is a semiconductor wafer. In another form of the invention, semiconductor layer  102  is a semiconductor chip. The etched openings  103  are formed over input/output (I/O) contact pads (not shown) on connection surface  101 . 
     In FIG. 1B, solder is vapor deposited on the photoresist-covered surface of semiconductor layer  102 . The vapor-deposited solder fills the etched cavities  103  in photoresist layer  100 , thereby forming solder columns  104  on the contact pads of connection surface  101 . In one embodiment, rather than using vapor deposition, solder is formed in etched cavities  103  by sputtering or electroplating. In one form of the invention, the solder is a low tin solder with a composition of about 90 percent lead and about 10 percent tin. After deposition of the solder, excess solder on the top surface of photoresist layer  100  is removed by a grinding process. 
     Next, as shown in FIG. 1C, photoresist layer  100  is removed, leaving a micro column grid array (CGA) of solder columns  104 . In one form of the invention, photoresist layer  100  is removed by etching. In one embodiment, solder columns  104  are generally cylindrical in shape, with a diameter of about 100-150 micrometers, a height of about 100-500 micrometers, and pitch of about 1000 micrometers, although virtually any pitch may be provided. After columns  104  have been formed, and photoresist layer  100  has been removed, the device is ready to mount to the next level of packaging as a true chip scale package (CSP). As shown in FIG. 1D, semiconductor layer  102  is directly connected to the next level of packaging  106  (e.g., circuit board or other substrate layer) with micro columns  104 . 
     It is desirable to have a solder column height of about 400-500 micrometers to provide an appropriate level of reliability. Typically, the taller the solder column, the greater the level of compliancy. In one embodiment, the process described above is capable of providing a column height of approximately 100 micrometers. To provide additional column height, the process described above is repeated as many times as necessary on top of the existing structure, until the desired column height is reached. 
     FIGS. 2A-2C are cross-sectional diagrams illustrating one embodiment of a process for extending the height of the columns in a micro column grid array according to the present invention. FIG. 2A illustrates a device or wafer in the process stage shown in FIG. 1B, with a second layer of photoresist  200  formed over the first layer of photoresist  100 . Photoresist layer  200  is etched to provide openings or cavities  203  positioned over the existing solder columns  104 . 
     In FIG. 2B, solder is vapor deposited on the device, thereby filling the etched cavities  203  in photoresist layer  200  and forming solder column extensions  204  on top of solder columns  104 . In one embodiment, rather than using vapor deposition, solder is formed in etched cavities  203  by sputtering or electroplating. After deposition of the solder, excess solder on the top surface of photoresist layer  200  is removed by a grinding process. 
     Next, as shown in FIG. 2C, photoresist layers  100  and  200  are removed, leaving a micro column grid array of solder columns  104  extended by solder column extensions  204 . In one form of the invention, photoresist layers  100  and  200  are removed by etching. In one embodiment, after the preferred column height is reached and the wafer-level processing is complete, semiconductor layer  102  is diced into individual semiconductor chips in a conventional manner. 
     FIGS. 3A-3E are cross-sectional diagrams illustrating an alternative embodiment of a process for forming a micro column grid array according to the present invention, including the formation of a protective layer that surrounds the micro columns. FIG. 3A illustrates a device or wafer in the process stage shown in FIG. 1B, with a second layer of photoresist  300  formed over the first layer of photoresist  100 . Photoresist layer  300  is etched to provide openings or cavities  303  positioned over the existing columns  104 . 
     In FIG. 3B, eutectic solder bumps  304  are vapor deposited in etched cavities  303  on top of solder columns  104 . In one embodiment, solder bumps  304  are about 63% lead and about 10% tin. Next, as shown in FIG. 3C, photoresist layers  100  and  300  are removed and solder bumps  304  are reflowed, leaving a micro column grid array of solder columns  104  with solder bumps  304  formed thereon. In one form of the invention, photoresist layers  100  and  300  are removed by etching. 
     Next, as illustrated in FIG. 3D, a protective layer  308  is applied to connection surface  101  of semiconductor layer  102 , and around micro columns  104 . In one embodiment, protective layer  308  is an elastic polymer. In another embodiment, protective layer  308  is a silicon-based material. 
     In FIG. 3E, solder bumps  304  are reflowed to attach semiconductor layer  102  to circuit board  312 . In one embodiment, solder bumps  304  have a different composition and a lower melting temperature than solder columns  104 . Protective layer  308  provides protection to semiconductor layer  102  and support to micro columns  104 , and makes the component more robust. Protective layer  308  preferably does not extend up to the entire height of columns  104 , as this would reduce the flexibility of columns  104 , and limit the compliancy columns  104  provide between semiconductor layer  102  and circuit board  312 . In one embodiment, protective layer  308  is applied while devices are still in wafer form to allow application to multiple devices at one time. 
     Embodiments of the present invention provide increased reliability and versatility, and allow chips to be placed into packaging configurations that were previously not considered feasible. Examples of configurations that may be provided include flip chip plastic packaging, direct mount of large, high I/O silicon chips onto an organic circuit board (Chip on Board), and attachment of chips to metal frames without the use of wirebonding. Additional applications include chip stacking, and conversion of existing wirebond die into flip chip mountable die. Through chip level processing according to embodiments of the present invention, micro column connections between a chip and the next level of packaging can be provided at a greater density than the connections in current flip chip designs. The micro columns also provide a level of reliability that allows greater versatility in chip packaging. By using chip/wafer level processes, metal columns can be deposited onto chip contacts with an ultra fine diameter and pitch. Multiple wafers may be processed at one time, thereby providing increased processing speed. 
     Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the chemical, mechanical, electro-mechanical, electrical, and computer arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.