Patent Application
20090155958
The subject innovation relates generally to die bonding systems and processes. It finds particular application in conjunction with light emitting diode (LED) dies, and will be described with particular reference thereto. However, it is to be appreciated that the systems and methods described herein are also amenable to other applications.
Conventional soldering die attachment processes (e.g., U.S. Pat. No. 6,222,207 B1; U.S. Pat. No. 6,593,160 B2) have limitations concerning flip chip bonding of LED vertical dies (e.g., such as Cree XB dies based on SiC substrates). Close proximity of the edge of the SiC substrate to metal on the bottom of the die can cause conductive path if residual die attachment material (such a solder) extends up the edge of the die and contacts the SiC (
Ultrasonic flip chip bonding does not use die attachment material, bonding is accomplished in short period of time, the reliability of the connections is high due to metal bonding (e.g., Au—Au solid phase diffusion), and the technique is lead free. Typically, successful flip chip Au—Au interconnect techniques use Au terminated bumps (Au plated or Au stud bumps fabricated on the LED die or sub mount side).
However, conventional ultrasonic bonding requires applying significant force to the LED shaped substrate, and these shear forces within the substrate often exceed a failure threshold for the substrate, resulting in cracks and die damage. A similar “force issue” takes place in the ultrasonic bonded sapphire-based AlInGaN dies, which typically use thinner (e.g., a total die thickness of approximately 3-4 mm) sapphire substrates. Thinning or eliminating the sapphire substrate can result in die performance improvement but it further exacerbates mechanical strength issues for ultrasonic bonding. Ultrasonic bonding can be facilitated using pre-heated sub mount wafers, but that requires a long-time exposure of the wafer to high temperatures, causing a degradation of the wafer and soldering material.
Thus, there exists a need for systems and/or methods that overcome the above-mentioned deficiencies and others.
According to one aspect, a die bonding system comprises a thermally conductive pickup tool that picks up a die, a heater that heats the pickup tool to a predetermined temperature, wherein the heated pickup tool heats a die attachment material that is employed to couple the die to a submount to a first temperature that is below the melting point of the die attachment material, and an ultrasonic transducer coupled to the pickup tool, wherein ultrasonic transducer heats the die attachment material to a second temperature that is equal to or greater than the melting point of the die attachment material.
According to another aspect, a method of bonding a die to a submount comprises positioning a die over a submount using a vacuum tool, conductively or remotely applying heat to the vacuum tool and conductively heating a die attachment material on the die to a first predetermined temperature, applying ultrasonic energy to the die to further heat the die attachment material to a second predetermined temperature, and applying a bonding force to the die to bond the die to the submount.
Yet another aspect relates to an apparatus for bonding a die to a submount, comprising means for positioning a die over a submount using a vacuum tool, means for applying heat to the vacuum tool and conductively heating a die attachment material on the die to a first predetermined temperature, means for applying ultrasonic energy to the die to further heat the die attachment material to a second predetermined temperature, and means for applying a bonding force to the die to bond the die to the submount.
Systems and methods are described herein, which facilitate reducing or eliminating excess die attachment material accrual and parasitic conductive paths formed in conjunction therewith by locally melting a die attachment material (e.g., solder) using a combination of localized heat sources and ultrasonic energy. The heat sources bring the die attachment material close to its melting point, which reduces an amount of bonding force required of purely ultrasonic bonding techniques. An ultrasonic transducer brings the die attachment material the rest of the way up to its melting point, which reduces the overall temperature that the die and/or sensitive components thereon endure during the bonding process.
With reference to
The die attachment layer 20, such as soldering material, couples the die 10 to a submount 24. Additionally, excess die attachment material 26 is shown, which has been squeezed out from beneath the die structure during a conventional bonding process. The excess die attachment material provides a parasitic conductive path 28, which can exhibit Schottky diode-like conductivity. Additionally, a distance d illustrates a predetermined distance between the bottom of the SiC layer 14 and the top of the submount 24. In one example, the predetermined distance is approximately 5 micrometers. As mentioned above, a gold-gold interconnect (GGI) can be employed if gold stud bumps or gold terminated bumps are fabricated on the die or submount side.
The following figures describe systems and methods for mitigating the formation of the parasitic conductive path 28 and accumulation of the excess die attachment material 26, which can be undesirably formed using conventional bonding techniques.
With reference to
The bonding process performed by the system 40 reduces and/or eliminates residual flux by locally melting die attachment material, or solder (not shown), which may be applied to the thermally conductive nodes and/or to the bottom of the substrate on the die at positions corresponding to the nodes. This in turn reduces the bonding pressure needed to bond the die to the submount, which reduces the risk of damage to the chip. For a wafer-level process, pre-bonded chips can be held at relatively low temperature because heat is applied to both the work holder and the pickup tool. Die attachment material is preliminary deposited onto the die and/or the conductive nodes, melted locally during a reflow stage, and thus localized within the interconnect area to prevent non-controlled spreading of die attachment material and/or to prevent unintended parasitic semiconductor structures or short circuits.
A miniature heater can be applied to the pickup tool to heat the pickup tool, at 84, and can heat the tool, to a temperature lower that the melting point of the solder applied to the bonding surface(s). In one example, the solder is a gold-tin (Au—Sn) alloy with a melting point of approximately 280° C. When the solder is near its melting point, the ultrasonic energy is applied to melt the solder locally, where the thermally conductive nodes on the submount form a mechanical interconnect with contact pads on the die. In one example, the ultrasonic energy is applied for approximately 0.5-2.0 seconds. The solder can be deposited on contact pads on the bottom of the die or on the submount nodes, or both. The bonding force magnitude is a function of the number of nodes on the submount, and can be on the order of approximately 200-800 grams, thereby significantly reducing an amount of force needed for conventional ultrasonic bonding techniques. Additionally, submount wafer or printed circuit board (in the chip-on-board case) can be kept at acceptable temperatures for InAlGaN-based dies during processing of a whole wafer (board).
As mentioned above, the bonding method thus combines ultrasonic and thermal energy to provide local soldering conditions for bumped submount and die. That is, ultrasonic energy provides an extra local source of heat to reach the solder melting point in the locations where Au plated nodes have mechanical contact with appropriate Au—Sn contact pads on the die side. It will be appreciated that other solder compositions can be used, including but not limited to silver-tin-copper (Ag—Sn—Cu) lead free solders, tin (Sn), etc.
Various embodiments and examples of the innovation have been described herein. It is appreciated that modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiments be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
