Critical plasma processing parameters for improved strength of wire bonds
BY J. GETTY, L. WOOD AND C. FAIRFIELD
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As integrated circuit (IC) packaging shrinks, the accompanying decrease in wire bond pad size causes an increased susceptibility to bond pad contamination. Wire bond pad contamination can result in poor bond pull strength and poor bond strength uniformity. Thus, it is particularly important to remove all contaminants from the bond pad surfaces prior to wire bonding.1,2,3,4,6 An efficient and cost-effective method for preparing the wire bond pads prior to wire bonding is with the use of radio-frequency-driven, low-pressure plasmas.5,6 The successful application of plasma technology relies on an optimization of process parameters including process pressure, plasma power, time and process gas type. These key plasma process parameters and their impact on wire bond pull strength are discussed.
Plasma Cleaning Technologies
Not all plasma technologies are the same, nor are all integrated circuit packages the same, making an understanding of plasma technology and integrated circuit packaging critical for successful results. In developing a successful plasma cleaning process for wire bond strength improvement, important factors include the substrate material, its chemical and temperature sensitivity, methods for handling the substrate, throughput and uniformity. Understanding these requirements ultimately defines the process parameters for the plasma system.
The objective of the plasma process is to maximize wire-pull strength, thereby minimizing failures and increasing yields. This must be achieved while minimizing the impact on the throughput of the packaging production line. It is therefore critical to optimize the plasma process through the judicious selection of process gas, operating pressure, time and plasma power. Incorrect selection of the process conditions can result in minimal improvement on the wire bond strength or even decreased wire bond strength.
 Figure 1. A typical plasma. |
A typical plasma consists of electrons, ions, free radicals and photons generated from the application of electromagnetic radiation to a gas volume at low pressure (Figure 1). There are various ways to produce plasma, but the preferred method is the use of radio frequency excitation. The highly energized non-equilibrium plasma is capable of surface cleaning and surface activation through physical, chemical and physical/chemical mechanisms without changing the bulk properties of the materials being cleaned. The degree of selectivity, anisotropy, uniformity and cleaning rates are a function of process parameter selection. The process parameters also dictate whether the process is physical, chemical or a combination of these mechanisms. Each have distinct advantages and disadvantages when it comes to cleaning bond pad sites. The selection of the process gas, the chamber pressure, the power applied and the time of the process determine the type of cleaning mechanism and its effects.
Plasma Process Parameters
Process gas: In a physical process, ions generated in an argon plasma bombard the surface with sufficient energy to remove contamination from the surface. The positively ionized argon atom will be attracted to the negatively charged electrode plate in the plasma chamber. This electrical attraction pulls the ion forcibly toward the electrode. As the ion impacts the bond pad surface, the impact force is sufficient to dislodge any contamination on the surface. This effluent is then removed via the vacuum pump.
The advantages of bombardment are that it is not a chemical reaction, and it cleans the surfaces of parts without leaving any oxidation. The ions are an important component in removing contaminates such as metal salts and other non-organic contaminates that are not easily removed through chemical processes. The end product is a surface made up entirely of the substrate material.2,4,6 Disadvantages include possible over etching of organic substrate material and re-deposition of contaminant or substrate particles onto other, undesirable areas. Nevertheless, these disadvantages are usually relatively easy to control by fine tuning the process parameters.
 Figure 2. Mean free path of argon as a function of pressure. |
Chemical processes use the plasma generated gas phase radicals to react with the compounds on the sample surface to produce gas phase by products that are subsequently pumped from the plasma system. For example, organic contamination is effectively removed with an oxygen plasma where the oxygen radicals react with contamination producing carbon dioxide, carbon monoxide and water. Cleaning speed and greater etching selectivity are the advantages of chemical cleaning in plasma, and as a general rule, chemical reactions will do a better job removing organic contamination.2,4,6 The main disadvantage arises from the fact that oxides can be produced on the surface of the substrate, and in many wire bonding applications, oxidation can be most undesirable.2,4,6 As in the case of bombardment, these disadvantages are relatively easy to control with the proper selection of the process parameters.
Pressure: Process chamber pressure is a function of the gas flow rate, and the product outgassing rate as well as the pumping speed. The selection of the process gas dictates the plasma cleaning mechanism (physical, chemical or physical/chemical) and, ultimately, the gas flow rate and process pressure regime.
Physical processes generally require lower pressures than chemical processes. Physical plasma cleaning requires that the energized particles impact the substrate surface before deactivation through collisional de-excitation. If the process pressure is high, the energized particle will experience large numbers of collisions with other particles prior to arrival at the bond pad, thus reducing its cleaning capability. The distance that the energized particle travels before a collision is known as the mean free path of the particle and is inversely proportional to pressure. The mean free path, l is defined as
l = (kT/s2P)
where P and T are the pressure and temperature of the gas, k is a constant and σ is the diameter of the gas molecule. Figure 1 displays the mean free path of argon as a function of pressure. Physical processes require low pressure in an effort to maximize the mean free path thus maximizing bombardment impact.
If the pressure is reduced significantly, however, there will not be sufficient reactive species available to clean the substrate in a reasonable amount of time.
Chemical processes rely on the chemical reaction of the plasma generated gas phase radicals with the substrate surface and use higher pressure. The use of higher process pressure in chemically reactive plasma processes is due to the need for a high concentration of reactive species at the substrate surface. Because of the higher pressure, chemical processes have faster cleaning rates.
 Figure 3. Wire bond failure as a result of poor bond pad surface preparation. |
Process pressure as a function of wire bond pull strength was evaluated for the physical cleaning of bond pads on PBGA substrates treated in magazines.1 For the high pressure/high flow (140-160 mTorr) conditions the pull strengths were not appreciably better than the untreated substrates (Figure 3). The lack of improvement is likely due to a short mean free path as a result of the high pressure, thereby reducing the number of energized particles reaching the substrate surface. Low pressure/low flow (100 mTorr) conditions also resulted in poor wire bond pull strengths due to an insufficient concentration of reactive species as result of the lower pressure.
Selecting an intermediate process pressure (120 mTorr) resulted in an improvement of wire bond pull strength of more than 20 percent. The results illustrate that although the general pressure regime is defined by the selection of the process gas, further refinement of the operating pressure is critical for optimal performance.
Power: Plasma power increases the cleaning rate by increasing the ion density and ion energy within the plasma. The ion density is the number of reactive species per unit volume. Maximizing the ion density will maximize the cleaning rate because of the relatively large concentration of reactive species. Ion energy defines the ability of the reactive species to perform physical work.
Plasma process power was evaluated for wire bond improvement. Increasing the power has a dramatic effect on wire bond improvement. For example, by increasing the power by a factor of two, the wire bond pull strength doubled. Increasing the power too much, however, can be detrimental to the substrate as well as ineffective to the process.
Time: In general, the objective is to minimize the process time in an effort to maximize the throughput of the packaging production line. Process time should be balanced with power, pressure and gas type. Evaluation of process time at optimum process pressure and power as a function of wire bond strength for PBGA type substrates demonstrates the importance of process time. In one example, a short plasma process time gave less than a two percent improvement on the wire bond strength vs. untreated substrates, where with an additional 28 percent increase in process time, the wire bond pull strength increased by more than 20 percent compared to the untreated substrates. Longer process run times do not always provide increased bond improvement results. The cleaning time required to achieve acceptable wire bond pull strength depends on the other process parameters.
Conclusions
Plasma cleaning of substrates before wire bonding has demonstrated a significant increase in wire bond pull strength when compared to non-plasma cleaned substrates. However, successful application of the technology requires an understanding of not only the substrate material, but also the effect of key plasma process parameters. Process gas, pressure, plasma power and process time must be carefully selected. Without proper optimization, the benefits of plasma cleaning cannot be fully realized.
AP
References
- L. Wood, C. Fairfield and K. Wang, “Plasma Cleaning of Chip Scale Packages for Improvement of Wire Bond Strength,” TAP, 2nd Edition, pp. 75-78 (2001).
- T. Liston, “Plasma Cleaning and Surface Treatment of Hybrids to Improve Bonding,” Circuit Expo 1986 Proceedings, pp. 41-43.
- Hutchinson Technology Inc., “Effect of Temperature, Contamination and Other Factors on Bond Strength,” TSA Suspension Termination Technical Issues, September 12, 1997.
- J. Nesheim, Hewlett Packard, Loveland Technology Center, Loveland, CO, The Effects of Ionic and Organic Contamination on Wirebond Reliability.
- S. Gore, “Degradation of Thick Film Gold Bondability Following Argon Plasma Cleaning,” ISHM 1992 Proceedings, pp. 737-742.
- H. Bonham and P. Plunkett, “Plasma Cleaning for Improved Wirebonding on Thin-Film Hybrids,” EP&P, February 1979, pp. 42-55.
J. GETTY is market development manager, L. WOOD is applications engineer, and C. FAIRFIELD is vice president of marketing and new business development at March Plasma Systems. For more information, contact Christa Fairfield at 4057 Port Chicago Highway, Concord, CA 94520; 925-827-1240; Fax: 925-827-1189; E-mail: [email protected].