After the final passivation layer and an alloy step in wafer fabrication, the circuits are complete. However, one or two additional processes may be performed on the wafer before transfer to packaging. These steps (wafer thinning and backside gold) are optional, depending on the wafer thickness and the particular circuit design.
Wafer thinning.
The trend to thicker wafers presents several problems in the packaging process. Thicker wafers require the more expensive complete saw-through method at die separation. While sawing produces a higher-quality die edge, the process is more expensive in time and consumption of diamond-tipped saws. Thicker die also require deeper die attach cavities, resulting in a more expensive package. Both of these undesirable results are avoided by thinning the wafers before die separation. Another situation requiring wafer thinning is electrical in nature. If the wafer backs are not protected as the wafers go through the dopant operations in fabrication, the dopants will form electrical junctions in the wafer back, which may interfere with good conduction in the back contact that is required for the circuit to operate correctly. These junctions may require physical removal by wafer thinning. The thinning step generally takes place between wafer sort and die separation. Wafers are reduced to a thickness of 0.2 to 0.5 mm.
Thinningis done by the same processes (mechanical grinding and chemical-mechanical polishing-CMP) used to grind wafers in the wafer preparation stage. An alternate method is to protect the front of the wafers and chemically etch material from the back. Wafer thinning is a worrisome process. In back grinding, there is the concern of scratching the front of the wafer and of wafer breakage. Since the wafer must be held down on the grinder or polishing surface, the front of the wafer must be protected and, once thinned, wafers are easier to break. In back etching, there is a similar need to protect the front of the wafer from the etchant. The protection can be provided by spinning a thick layer of photoresist on the front side. Other methods include attaching adhesive-backed polymer sheets cut to the wafer diameter. Stresses induced in the wafer by grinding/polishing processes must be controlled to prevent wafer and die warping. Wafer warping interferes with the die separation process (broken and cracked die). Die warping creates die attach problems in the packaging process.
Backside gold.
Another optional wafer process is adding a layer of backside gold. A layer of gold is required on wafers that are going to be Packaging attached to the package by eutectic techniques. The gold is usually applied in the fabrication area (after back grinding) by evaporation or sputtering.
Die separation
The chip-packaging process starts with the separation of the wafer into individual dies. The two methods of die separation are scribing and sawing
Scribing, or diamond scribing, was the first production die separation technique developed in the industry. It requires dragging a diamond-tipped scribe through the center of the scribe lines and separating the die by flexing the wafer. Scribing becomes unreliable in wafers over 10 mils thick.
The advent of thicker wafers has led to the development of sawing as the preferred die separation method. A saw consists of a wafer table with rotation capability, a manual or automatic vision system for orienting the scribe lines, and a diamond-impregnated round saw. Two techniques are used. Both start with the passage of the diamond saw over the scribe lines. For thinner wafers, the saw is lowered into the wafer surface to create a trench about one-third of the way through the wafer. The separation of the wafer into die is completed by the stress and roller technique used in the scribing method. The second sawing method is to separate the die by a complete saw through of the wafer. Often, the wafers for complete saw-through are first mounted on a flexible plastic film. The film holds the die in place after the sawing operation and aids the die pick operation. Sawing is the preferred die separation method due to the cleaner die edges and the fewer cracks and chips left on the sides of the die.
Die pick and place
After sawing, the separated die are transferred to a station for selection of the functioning die (non-inked). In the manual method, an operator will pick up each of the non-inked dies with a vacuum wand and place it in a sectioned plate. Wafers that come to the station on the flexible film are first placed on a frame that stretches the film. The stretching separates the die, which aids the die pick operation. In the automated version of this operation, a memory tape or disk that has the locations of the good die (from wafer sort) is loaded into the tool. A vacuum wand picks up good die and automatically places them in a sectioned plate for transfer to the next operation.
Before being committed to the rest of the process, the die are given an optical inspection. Of primary interest is the quality of the die edge, which should be free of chips and cracks. This inspection also sorts out surface irregularities, such as scratches and contamination. Inspection may be manual with microscopes or automated with a vision system. At this step, the die is ready to go into a package.
Die attachment has several goals: to create a strong physical bond between the chip and the package, to provide either an electrical conducting or insulating contact between the die and the package, and to serve as a medium to transfer heat from the chip to the package. A requirement is the permanency of the die-attachment bond. The bond should not loosen or deteriorate during subsequent processing steps or when the package is in use in an electronic product. This requirement is especially important for packages that will be subjected to high physical forces, such as in rockets. Additionally, the die attach materials should be contaminant-free and should not outgas during subsequent process heating steps. Lastly, the process itself should be productive and economical.
There are two principal methods of die attach: eutectic and epoxy adhesives. The eutectic method is named for the phenomenon that takes place when two materials melt together (alloy) at a much lower temperature than either of them separately. For die attach, the two eutectic materials are gold and silicon. Gold melts at 1063°C, while silicon melts at 1415°C. When the two are mixed together, they start alloying at about 380°C. Gold is plated onto the die-attach area and alloys with the bottom of the silicon die when heated. The gold for the die-attach layer is actually a sandwich. The bottom of the die-attach area is deposited or plated with a layer of gold. Sometimes, a preformed piece of metal composed of a gold and silicon mixture is placed in the die-attach area. When heated, these two layers, along with a thin layer of silicon from the wafer back, form a thin alloy layer. This layer is the actual bond forming the die-package attachment. Eutectic die attach requires four actions. First is the heating of the package until the gold-silicon forms a liquid. Second is the placement of the chip on the die-attach area. Third is an abrasive action, called scrubbing , that forces the die and package surfaces together. It is this action, in the presence of the heat, that forms the gold-silicon eutectic layer. The fourth and final action is the cooling of the system, which completes the physical and electrical attachment of the chip and package. Eutectic die attach can be performed manually or by an automated machine that performs the four actions. Gold-silicon eutectic die attach is favored for high-reliability devices and circuits for its strong bonds, heat dissipation properties, thermal stability, and lack of impurities.
The alternate die-attach process uses thick liquid epoxy adhesives. These adhesives can form an insulating barrier between the chip and package or become electrically and heat conductive with the addition of metals such as gold or silver. Polyimide may also be used as an adhesive. Popular adhesives are silver-filled epoxy for copper lead frames and silver-filled polyimide for Alloy 22 metal frames. The epoxy process starts with the deposit of the epoxy adhesive in the die-attach area by dispensing the adhesive with a needle or screen printing it into the die-attach area. The die, held by a vacuum wand, is positioned in the center of the die-attach area. The second action is to force the die into the epoxy to form a thin uniform layer under the die. The final action is a curing step in an oven at an elevated temperature that sets the epoxy bond. Epoxy die attach is favored for its economy and ease of processing, in that the package does not have to be heated on a stage. This factor makes the automation of the process easier. When compared to goldsilicon eutectic die attach, epoxy has the disadvantage of potential decomposition at the high temperatures of bonding and sealing operations. Epoxy die-attach films also do not have the bonding power of the eutectics. Regardless of the attachment method used, there are several marks of a successful die attach. One is the proper and consistent alignment of the die in the die-attach area. Proper placement pays off in faster and higher-yield automatic bonding. Another desired result is a solid, uniform, and void-free contact over the entire area of the chip. This is necessary for mechanical strength and good thermal conduction. One evidence of a uniform bond is a continuous joint or “fillet” between the die edge and the package. The final mark of a good die-attach process is a die-attach area free of flakes or lumps that can come loose during use and cause a malfunction.
Once the die and package are attached, they go to the bonding process. This is perhaps the most critical of all the assembly operations. Three techniques provide the critical chip/package connection: wire bonding, bump/flip-chip, and TAB. In wire bonding, up to hundreds of wires must be perfectly bonded from the bonding pads to the package inner leads. In bump/flip chip, the bonding pad/package connection is a solder ball. Tape automated bonding (TAB) system is a process that bonds the lead frame leads directly to the die bonding pads in one step.
The wire bonding procedure is simple in concept. A thin (0.7 to 1.0 mil) wire is first bonded to the chip bonding pad and spanned to the inner lead of the package lead frame. The third action is to bond the wire to the inner lead. Last, the wire is clipped and the entire process repeated at the next bonding pad. While simple in concept and procedure, wire bonding is critical because of the precise wire placement and electrical contact requirements. In addition to accurate placement, each and every wire must make good electrical contact at both ends, span between the pad and inner lead in a prescribed loop without kinks, and be at a safe distance from neighboring wires. Wire loops in conventional packages are 8 to 12 mils, while those in ultra-thin packages are 4 to 5 mils. Distances between adjacent wires are referred to as the pitch of the bonding. Wire bonding is done with either gold or aluminum wires. Both are highly conductive and ductile enough to withstand deformation during the bonding steps and still remain strong and reliable. Each has its advantages and disadvantages, and each is bonded by different methods.
Gold has several pluses as a bonding wire material. It is the best known room-temperature conductor and is an excellent heat conductor. It is resistant to oxidation and corrosion, which translates into an ability to be melted to form a strong bond with the aluminum bonding pads without oxidizing during the process. Two methods are used for gold bonding. They are thermocompression and thermosonic.
Thermocompression bonding (also known as TC bonding) starts with the positioning of the package on the bonding chuck and the heating of the chip and package to between 300 and 350°C. Chips that are going to be enclosed in an epoxy molded package are processed through die attach and bonded with the chip on the lead frame only. The bonding wire is fed out of a thin tube called a capillary. An instantaneous electrical spark or small hydrogen flame melts the tip of the wire into a ball and positions the wire over the first bonding pad. The capillary moves downward, forcing the melted ball onto the center of the bonding pad. The effect of the heat (thermal) and the downward pressure (compression) forms a strong alloy bond between the two materials. This type of bonding is often called ball bonding. After the ball bond is complete, the capillary feeds out more wire as it travels to the inner lead. At the inner lead, the capillary again travels downward to where the gold wire is forced by the heat and pressure to melt onto the gold-plated inner lead. The spark or flame then severs the wire, forming the ball for the next pad bond. This procedure is repeated until every pad and its corresponding inner pad are connected.
Thermosonic, gold ball bonding follows the same steps as thermo-compression bonding. However, it can take place at a lower temperature. This benefit is provided by a pulse of ultrasonic energy that is sent through the capillary into the wire. This additional energy is sufficient to provide the heat and friction to form a strong alloy bond. The majority of production gold wire bonding is done on automatic machines that use sophisticated techniques to locate the pads and span the wire to the correct inner lead. The fastest bonding machines can perform thousands of bonds per hour. There are two major drawbacks to the use of gold bonding wires. First is the expense of the gold. Second is an undesirable alloy that can form between the gold and aluminum. This alloy can severely reduce the conduction ability of the bond. It forms a purplish color and is known as the “purple plague.”
Aluminum wire bonding.
Aluminum wire, while not having the conduction and corrosion-resistance properties of gold, is still an important bonding wire material. A primary advantage is its lower cost. The second advantage is that the bond with the aluminum bonding pad is a monometal system and thus less susceptible to corrosion. Also, aluminum bonding can take place at lower temperatures than gold bonding, making it more compatible with the use of epoxy die-attach adhesives. The bonding of the aluminum follows the same major steps as gold wire bonding. However, the method of forming the bond is different. No ball is formed. Instead, after the aluminum wire is positioned over the bonding pad, a wedge forces the wire onto the pad as a pulse of ultrasonic energy is sent down the wedge to form the bond. After the bond is formed, the wire is spanned to the inner lead where another ultrasonic-assisted wedge bond is formed. This type of bonding is known variously as ultrasonic or wedge bonding. After this bond, the wire is cut. At this point in the process, a major difference between the bonding of the two materials occurs. In gold bonding, the capillary moves freely from pad to inner lead, to pad, and so forth, with the package in a fixed position. In aluminum wire bonding, the package must be repositioned for every single bonding step. The repositioning is necessary to line up the pad and inner lead along the direction of travel of the wedge and wire. This requirement places an additional difficulty on the designers of automatic aluminum bonding machines. Nevertheless, most production aluminum bonding is done on high-speed machines.
Wire bonding presents several problems. There are electrical resistances associated with each bond. There are minimum height limits imposed by the required wire loops. There is the chance of electrical performance problems or shorting if the wires come to close to each other. Plus, the wires require an individual bonding step at both the chip bonding pad and at the package lead. Perhaps the biggest problem results from the increasing number of connections (pin count) needed to operate larger circuits. Chip designers simply run out of space to locate the required number of connections around the periphery of the chip. These issues are addressed by replacing wires with a deposited metal bump on each bonding pad. The bumps are also called balls, as in naming packages using bump/flip-chip processes as ball grid arrays (BGAs). This bonding method allows chip design with bonding pads located both along the edge of the die as well as in the interior of the die. These locations place the bump closer to the chip circuitry, increasing signal processing speed. Connection to the package is made when the chip is flipped over and the bump soldered to a corresponding package inner lead on a package or printed circuit board. IBM calls their version of this technology controlled collapse chip connection. This process leaves the die suspended above the package surface. Physical stresses and strains are absorbed by the soft solder bump. Additional stress tolerance is provided by filling the gap with an epoxy filling, called and underfill. Bump connection technology starts in the wafer fabrication process. Wafers are processed through the usual metallization, passivation, and bonding pad patterning processes. The last patterning process leaves an opening in the passivation layer over the bonding pads. A number of process flows are available to form the solder bumps on the bonding pad. The process described below is an example.
Sputter deposit intermetal stack.
Lead/tin solder balls are the preferred “bump” material. However, an intermetal layer (or stack) is required between the bonding pad and the solder ball to prevent the lead from diffusing into the aluminum pad and to aid adhesion of the solder ball onto the pad. Various metal stacks are used, including chrome-coppergold (Cr-Cu-Au), titanium-nickel (Ti-Ni), and plain copper.
Patterning step of bump location.
A patterning step covers the die surface with resist, leaving openings over the bonding pads and surrounding dielectric. The resist layer is thick enough to accommodate enough solder to form a ball of sufficient volume to provide structural support and lower electrical resistance between the chip and package or substrate.
Deposit of intermediate layer stack.
The intermetal materials are evaporated or sputtered through the openings on the pad. Deposit lead/tin solder. Deposit of the lead/tin solder can be by electroplating or evaporation. If electroplating is used, a seed layer is deposited before the electroplating. Lead/tin is used to lower the melting point of the solder. Remove resist. The resist is removed, leaving a bump of lead/tin connected
to the bonding pad.
to the bonding pad.
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