The present invention relates to an improved method and apparatus for semiconductor wafer bonding, and more particularly to an improved industrial-scale semiconductor wafer bonding operation that combines wafer surface treatment followed by direct wafer bonding.
Wafer-to-wafer (W2W) bonding is deployed in a wide range of semiconductor process applications for forming semiconductor devices. Examples of semiconductor process applications where wafer-to-wafer bonding is applied include substrate engineering and fabrication of integrated circuits, packaging and encapsulation of micro-electro-mechanical-systems (MEMS) and stacking of many processed layers (3D-integration) of pure microelectronics. The quality of the wafer-to-wafer bond affects the overall processing yield and manufacturing cost of these devices and ultimately the cost of the electronic products that incorporate these devices.
There are a number of wafer-to-wafer bonding methods. Of interest here are those methods that depend on the wafer surface metal structures as the joining adhesive. Examples of these metal structures are copper, gold, or aluminum pads and lines. In many cases one or both wafers will have opposing metal structures where at least one wafer will carry metal solder to act as an adhesive at the joining points. In other cases, the metal structures themselves are welded to join the wafers. In all metal joining methods, the joining metals must be free of oxides and organic contamination to ensure a strong bond. Two methods for W2W bonding using metal joining are direct wafer bonding and thermocompression bonding.
Direct wafer bonding refers to a process where two separate wafer surfaces are brought into contact and are bonded without any intermediate adhesives or external force. The initial bond strength is usually weak, and therefore a subsequent annealing step is generally carried out to strengthen the bond. The direct wafer bonding process can be viewed as a three-step process, including surface activation, room temperature bonding and annealing. The room temperature bonding, also known as pre-bonding is based on inter-atomic and intermolecular forces, also known as Van-der-Waals forces, hydrogen or water bridges. These forces are relatively weak. However, in many cases, a spontaneous bonding of two clean and flat surfaces occurs when initiated only in one single point. Typically the bonding is initiated in the center or at the edge. Once the bonding is initiated a so-called bonding front propagates across the bonding interface, as shown in the IR images of
Thermocompression bonding joins two wafers by using force and pressure to solder or weld the opposing metal structures of the wafer pair.
Several pretreatment methods of the bond surfaces have been suggested. However, most of the suggested methods are either not efficient or scalable to accommodate large scale semiconductor manufacturing processes. Accordingly, there is a need for an industrial-scale semiconductor wafer bonding operation that combines wafer surface treatment followed by direct wafer bonding.
In general, in one aspect, the invention features an improved apparatus for bonding semiconductor structures comprising equipment for treating a first surface of a first semiconductor structure and a first surface of a second semiconductor structure with formic acid, equipment for positioning the first surface of the first semiconductor structure directly opposite and in contact with the first surface of the second semiconductor structure and equipment for forming a bond interface between the treated first surfaces of the first and second semiconductor structures by pressing the first and second semiconductor structures together.
Implementations of this aspect of the invention may include one or more of the following features. The equipment for treating the surfaces of the first and second semiconductor structures with formic acid includes a sealed tank filled partially with liquid formic acid and partially with formic acid vapor. The tank includes an inlet valve and an outlet valve. Opening the inlet valve connects the tank to a nitrogen gas source and allows nitrogen gas to flow through the tank. Opening the outlet valve allows a mixture of formic acid vapor with nitrogen gas to flow out of the tank and the mixture is then used for treating the surfaces of the first and second semiconductor structures. The mixture is adjusted to comprise 4% of formic acid. The equipment for treating the surfaces of the first and second semiconductor structures with formic acid further includes a leak detector for detecting low levels of formic acid vapor outside of the formic acid treatment equipment. The equipment for treating the surfaces of the first and second semiconductor structures with formic acid further includes a nitrogen gas pressure sensor, a pressure monitor device, a nitrogen gas pressure regulator, a nitrogen gas flow meter, a gas control valve configured to be materially compatible with formic acid, an error indicator for indicating nitrogen gas pressure below a set value, and an electrical lock-out switch configured to shut off electrical power to the equipment in cases where an error is indicated or a formic acid leak is detected. The tank further comprises a tank pressure gauge monitoring pressure inside the tank, and high and low formic acid level sensors indicating the fill level of the liquid formic acid in the tank. The equipment for treating the surfaces of the first and second semiconductor structures with formic acid is made of materials compatible with formic acid. The equipment for treating the surfaces of the first and second semiconductor structures with formic acid further comprises means for preventing tipping of the equipment, a first enclosure cabinet and a second enclosure cabinet. The first enclosure cabinet encloses the tank, the tank pressure gauge, the tank inlet and outlet valves, a tank bypass valve, a tank shut-off valve, and the high and low formic acid level sensors. The second enclosure cabinet encloses the nitrogen gas pressure sensor, the pressure monitor device, the nitrogen gas pressure regulator, the nitrogen gas flow meter, the gas control valve, the error indicator, the electrical lock-out switch, a status indicator, and a purge valve. The equipment for treating the surfaces of the first and second semiconductor structures with formic acid further comprises at least one tubing connecting the tank to the positioning and/or bonding equipment. All connections between the tubing and the tank are enclosed in the first enclosure cabinet.
In general, in another aspect, the invention features an improved method for bonding semiconductor structures comprising treating a first surface of a first semiconductor structure and a first surface of a second semiconductor structure with formic acid, positioning the first surface of the first semiconductor structure directly opposite and in contact with the first surface of the second semiconductor structure and forming a bond interface between the treated first surfaces of the first and second semiconductor structures by pressing the first and second semiconductor structures together. The formic acid is provided by a sealed tank filled partially with liquid formic acid and partially with formic acid vapor. The tank comprises an inlet valve and an outlet valve. Opening the inlet valve connects the tank to a nitrogen gas source and allows nitrogen gas to flow through the tank. Opening the outlet valve allows a mixture of formic acid vapor with nitrogen gas to flow out of the tank for treating the surfaces of the first and second semiconductor structures.
Referring to the figures, wherein like numerals represent like parts throughout the several views:
In a direct bonding process typically the wafers are oriented horizontally, as shown in
Direct bonding of silicon wafers requires smooth wafer surfaces both on the macroscopic and microscopic level. These requirements translate macroscopically into a surface bow of less than 40 micrometers and total indicated runout (TIR) of less than 2 micrometers and microscopically into surface roughness of root mean square (RMS) of less than 2 micrometers. These requirements are very difficult to attain in large scale manufacturing processes. As a result, extensive planarization steps via CMP and high forces during bonding are usually required to avoid surface defects, voids, and trapped gases at the bond interface.
Furthermore, metal-to-metal bonds, such as Cu—Cu or Al—Al bonding are affected by surface oxidation, as well as grain size control. Grain boundaries contribute to an increase in metal diffusion along the grain boundaries and result in increased bonding throughput. On the other hand, oxidation of the metal surfaces results in layers of surface oxides that need to be “cracked” to allow diffusion of metals to the bond interface. This is particularly true in the case of Al—Al bond where surface oxidation occurs even at room temperature and vacuum environment. As a result, a large force needs to be applied to “crack” the oxide layers. The applied force is usually 2-3 times higher than the force needed for bonding pure metals. The same oxide formation occurs on Cu-surfaces. However, copper oxide is soluble in copper and although it requires cleaning and passivation it does not require a very large force to be applied. Oxide removal prior to the bonding process reduces the required bond force and increases the bond yield to about 100%. Accordingly, oxide removal prior to bonding is advantageous in direct silicon-to-silicon and metal-to-metal bonds.
In one embodiment, formic acid is used to remove any surface oxides prior to the bonding process, as shown in
Formic acid (or methanoic acid) is a carboxylic acid that occurs naturally in the venom of bees and ant stings. At the industrial level, formic acid is produced as a byproduct in the manufacture of other chemicals such as acetic acid or is synthesized by reacting methanol with carbon monoxide at elevated temperature and pressure (80° C. and 40 atm). Safety is a major concern both with the production method and with formic acid itself. The principal danger from formic acid is due to exposure of skin or eye to liquid formic acid or contact with the concentrated vapors. Any of these exposure routes can cause severe chemical burns, and eye exposure can result in permanent eye damage. Inhaled vapors may similarly cause irritation or burns in the respiratory tract. Since carbon monoxide may also be present in formic acid vapors, care should be taken wherever large quantities of formic acid fumes are present. The US OSHA Permissible Exposure Level (PEL) of formic acid vapor in the work environment is 5 parts per million parts of air (ppm). Formic acid is readily metabolized and eliminated by the body. Nonetheless, some chronic effects have been documented, including allergies, liver or kidney damage. Accordingly, there is a need for a safe method of delivering formic acid in the oxide removal chamber.
Referring to
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Given the hazardous nature of formic acid the bubbler includes the following safeguards. Acid vapor detector 409 on the exhaust line 145, loss of exhaust pressure detector, non formic compatible components in a separate cabinet, loss of nitrogen pressure switch, manual lockout for each chamber and liquid leak detector in formic gas cabinet. Furthermore, the bubbler is mechanically secured to prevent tipping. In one example, the legs of the bubbler are bolted to the floor to prevent tipping.
In other embodiments, the oxide removal occurs in the same chamber where the bonding takes place. In either case, the bubbler delivers the formic acid aerosol to the process and or bonding chamber via stainless steel tubing. There are no connections in the line between the process/bonding chamber and the bubbler. All connections are located in the exhaust area.
Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
This application claims the benefit of U.S. provisional application Ser. No. 61/060,531 filed Jun. 11, 2008 and entitled “IMPROVED METHOD AND APPARATUS FOR WAFER BONDING”, the contents of which are expressly incorporated herein by reference.
Number | Date | Country | |
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61060531 | Jun 2008 | US |