Packaging for Higher Yield



Many factors in semiconductor manufacturing have contributed to an increased need for protection of wafers, singulated die or ICs during the back-end processes. Wafers, for example, continue to become thinner but have increased in size from 200 to 300 mm. They are also more fragile than previous wafers. As line widths and die bond pad sizes continue to shrink, contamination and damage in the assembly process becomes much less tolerable as well. At the same time, companies are facing increased pressure to speed up production, reduce waste and create smaller chips with more functionality.

Every manufacturer performs back-end functions differently, based on factors such as different business operations, geography and the product being manufactured. Despite these differences, there are some basic packaging, handling and transportation issues that can have a significant impact on yield, productivity and return on investment. The most critical issue is to understand and limit challenges in material integrity management such as physical, chemical or electrical damage to wafers, chips and packaged devices. To understand and combat these issues and maintain high yields, semiconductor companies need to examine three key areas of the back-end process to ensure that components are protected at each step with the right media and materials. These areas include: 1) from fab to assembly; 2) during assembly to test; and 3) during final test and shipping. This not only optimizes yield, but also provides lower cost of ownership.

Fab to Assembly

Once wafers leave the manufacturing process, they are exposed to new environments and are prone to physical, chemical and environmental damage.

Ionic/metal contamination. During the manufacture of wafer shippers and various resin-based materials that go into shippers, resins can contaminate the die or wafer when cations, anions and metals that leach out of the materials are absorbed into the die pad. When using shippers and other materials where ionic contamination is possible, it is important that a supplier thoroughly test and obtain specifications for cation, anion and metal values, and have manufacturing processes in place to maintain material integrity. With most materials, some level of cations, anions or other metals will show up in the testing report (depending on the reliability of the testing equipment and operator skills). Testing can be done for cations, including: ammonium, sodium, potassium, magnesium and calcium. Testing can also be done for the following anions: acetate, fluoride, chloride, nitrate/nitrite iron, bromide, sulfate and phthalate. Metals that can be tested for include: aluminum, copper, lead, tin and zinc.

Figure 1. Wafer shippers that have undergone testing for low levels of outgassing and ionics help maintain the integrity of wafers during shipping.
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By using a resin with the lowest cation/anion value, waste can easily be decreased from ionic contamination. Many suppliers do not offer testing capabilities or incoming specifications to measure against. Be sure to request testing results, not just values. Testing procedures are highly sensitive and greatly influence results. Temperature and the time of testing can affect final results, so it is important to know these conditions.

Outgassing. Outgassing is the release of organic compounds, which can happen when finished wafers are packaged in a closed wafer shipper container (Figure 1). Low levels of outgassing can be harmless, but must be monitored to prevent contamination and corrosion. In some cases, condensation of an airborne element released from a closed shipper can be absorbed into the surface/bonding pads of a wafer and cause contamination.

There are several methods suppliers can use to measure the outgassing behavior of materials to ensure that a contamination factor does not exist. The two most common are dynamic, non-equilibrium headspace (samples must fit in 10- to 20-mL vials) and static equilibrium headspace (used for whole part testing) analysis.

Temperature is critical in both testing modes. Test methods range in temperature from 23 to 100°C, or greater. Testing time can also vary from minutes to days. In the case of whole part testing, testing is usually completed at room temperature for three or more days. In the case of non-whole part testing, temperature and time vary. When discussing this with a supplier, it is extremely important to understand how outgassing tests were conducted — especially temperature, time and detection limits.

In most cases, a “total” outgassing value is reported. However, instances where only the outgassing of compounds between C8HY-C30HY have been reported. End users are beginning to set limits on these outgassing values, since they have been able to link a loss in wafers to high outgassing values of certain materials (organic compounds with specific carbon chain lengths).

Figure 2. Matrix trays are used to transport packaged ICs between assembly operations, as well as from assembly to test handlers or end users.
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Physical damage. Physical damage includes cracks or chips to wafers, bare die and packaged chips that can be seen by the human eye, as well as damage caused by corrosion, stress or opened/shorted circuits that cannot be seen by the human eye.

As electronic components in the semiconductor industry shrink and become more complex, so do methods of packaging and shipping. Packaging media must be able to hold finished wafers with little rotation or movement during handling. Old-style packaging designed for different or larger products simply cannot protect products that it was not designed for. New packaging provides better protection from contaminants and physical damage.

If products are packaged and shipped correctly during transport from manufacturing to assembly, companies can decrease loss and quickly recover investment. Companies can further lower total cost of ownership by reusing packaging and shipping materials. Some suppliers offer a program to provide cleaning, inspection, redistribution and recycling services. This cuts costs and allows suppliers to inspect shippers regularly to check for damage — ensuring wafer integrity.

Assembly to Test

During assembly, wafers are separated into dies and assembled into a final package through a series of processes that can cause physical damage and contamination if handled improperly. Most IC assembly practices use Joint Electronic Device Engineering Council-style matrix trays to transport packaged ICs between assembly operations and from assembly to test handlers or end-users.

Matrix trays are designed to hold as many components as possible to reduce the cost per unit and to maximize use of the trays (Figure 2). Trays are well accepted and widely used in assembly. As technology advances, die sizes tend to get smaller. Each time a die shrink occurs, it can potentially affect the final package size — as with some ball grid array packages — and make previous matrix trays obsolete. Accurate placement of packaged devices in trays is critical to protect the component and its fragile lead or ball contacts.

Final Test and Shipping

After assembly, components undergo rigorous testing to ensure they perform as expected. The way ICs are transported through the testing process can significantly impact yield.

Either trays or carrier tape are used in a variety of semiconductor and electromechanical application where electrostatic discharge-safe and nonelectrostatic discharge-safe materials are required. Carrier tape may be preferred for faster assembly by the end customer.

Regardless of transportation type, trays and tape and reel products should ensure that movement of components is limited to prevent physical damage and contamination from chemical transmission.

If components are to be stored, it is important to review storage requirements. For carrier tape, functions like age testing (to determine the integrity of materials after long-term storage) and process windows should be evaluated. It is also important that the tray or tape and reel product's materials used in the storage phase do not contaminate the final product by releasing organic or inorganic anions, which greatly reduces the yield of the stored product.

Overall Impact

By evaluating these key areas, companies can improve aspects of the process and increase yield, productivity and return on investment.

In the semiconductor industry, a small improvement in yield often results in huge financial improvements. In many cases, the decision to replace old media is quickly recovered in yield improvements. For example, a fab that produces 20,000 wafers per month could improve yield by one percent through better protection. If each wafer generates $5,000 in revenue, that one percent equals $1 million of additional revenue per month. Understanding each part of the finished wafer and packaged IC process and applying the proper handling and shipping media with the right supplier support is the key to maintaining post-fab component integrity.

RALPH HENDERER, vice president; LYNN MILBRETT, engineering program manager; and FRANK MANGANIELLO, senior materials engineer, may be contacted at Entegris Inc., 3500 Lyman Blvd., Chaska, MN 55318; (952) 556-3131; e-mail: [email protected], [email protected] and [email protected].


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