Story #3: Direct Immersion Gold as a Final Finish
Story #4: Surface Finishing for Lead-Free
Story #7: Surface Finishes in a Lead Free World
Story #14: Electroless Plating for LTCC Metallization
By Don Gudeczaukas
Under Bump Metallization (UBM) using Electroless Nickel / Immersion Gold (ENIG) is a low cost option for deposition of UBM layers. Unlike dry methods involving vacuum techniques, Uyemura under bump metallization technology uses wet processes and offers economic and throughput advantages. Throughput advantages are realized by batch processing of several wafers simultaneously while processing in cassette trays. Use of a dedicated (usually automated) tool for ENIG deposition can yield throughput of up to several hundred wafers per shift, depending on the tool size. The selectivity of the process allows for deposition of the ENIG only on the exposed aluminum or copper pads.
Wafer manufacturers are continuously searching for various methods to increase throughput and decrease wafer processing costs. Of note, the final layer of many integrated circuit bond pads consists of aluminum or copper. Aluminum can serve as an acceptable surface for standard wire bonding since the wire bonding techniques form an acceptable wire bond through the tenacious oxide layer normally present on the aluminum surface, but this surface is not acceptable for soldering or conductivity. As a result, under bump metallization methods are used to form a good bond to the aluminum pad and help prevent diffusion of metals into the IC itself.
Traditionally, dry methods are used to form the UBM (sputtering or vacuum deposition), but these methods require large capital equipment costs and results in relatively low productivity. These methods are, however, industry accepted and dry metallization techniques are still used for the majority of wafers. Wet chemical methods such as ENIG have been investigated to serve as both a solderable surface for the bumps themselves and as a wire bondable surface while increasing productivity.
The standard processing sequence for ENIG as an under bump metallization process for aluminum pads is shown in Figure 1.
Several challenges present themselves when attempting to chemically deposit ENIG for under bump metallization. The thickness of the aluminum pad, alloy content, and pad size play a significant role in the pre-plate conditions. Etching and zincate conditions may have to be modified based on the above conditions. An explanation of each process step is given below.
The cleaner ensures the removal of contamination from the surface prior to chemical treatment and wets the surface. Surface contamination in wafer handling environments (such as oils or fingerprints) is rare, but the cleaner helps make certain that the aluminum surface is clean and wetted for subsequent steps.
The micro-etching process removes or minimizes the oxide layer on the aluminum surface. This process step is used to obtain a more uniform zincate coating in subsequent steps. The type of aluminum alloy and thickness of the aluminum dictates whether this process step can be used and for how long.
1st Nitric Acid Dip
This step forms a uniform, thin oxide layer on the aluminum pad. The thin oxide layer assists the subsequent zincate process to properly deposit onto the aluminum.
The zincate process dissolves the aluminum oxide from the surface and deposits zinc by a galvanic reaction. Zinc coatings deposited from zincate solutions are very adherent and serve as an excellent seed layer for electroless nickel plating. Zinc is deposited onto the aluminum since the electroless nickel process is not catalytic to aluminum itself. Electroless nickel will readily deposit onto zinc surfaces, thus the requirement for this process step.
2nd Nitric Acid Dip
If the aluminum layer is thick enough, the initial zincate coating is stripped off in this process step and another thin oxide coating is deposited onto the aluminum.
A second layer of zinc is deposited. Generally, a second zincate layer is preferred as the zinc coating from the second layer is more uniform than from just one zincate layer. Thin aluminum coatings less than 0.5 microns may preclude the use of two zincates. It is very important that the underlying aluminum layer is not over-attacked during the process of zincating, otherwise functionality of the deposit may be compromised as shown in Figure 2.
Figure 2 Cross Section after Zincating
Additionally, the zincating process should deposit a uniform zincate coating on the surface as shown in Figure 3.
The electroless nickel is an autocatalytic process which uses a reducing agent (commonly hypophosphite) to chemically deposit a nickel-phosphorus alloy onto the zincated aluminum. Electroless nickel serves many functions such as:
Generally, at least 2 microns of nickel are deposited for under bump metallization applications, but some variations are seen based on special requirements such as deposit stress and electrical resistivity.
The immersion gold reacts galvanically with the electroless nickel to deposit a thin layer of gold onto the nickel. For each atom of nickel dissolved into solution, 2 atoms of gold are deposited onto the surface through the galvanic reaction. The gold layer helps prevent the electroless nickel from oxidizing and preserves solderability of the nickel surface. In fact, during soldering, the immersion gold layer typically dissolves into the solder as the solder joint if formed with the nickel. It is important that the immersion gold layer not corrode or damage the electroless nickel layer as the gold deposits, otherwise solderability of the under bump metallization layer may be compromised. A cross sectional view of an optimized ENIG deposit is shown in Figure 4. No corrosion of the nickel from the immersion gold reaction is detected. Optionally, an autocatalytic electroless gold can be applied over the immersion gold when gold wire bonding is required. In some cases gold wire bonding can be successfully performed on an ENIG deposit without the need to additional gold deposits from an electroless gold process.
Functional Testing of the ENIG Deposit
Solderability, or solder joint integrity, of the ENIG deposit can be measured in several ways. For this discussion, ball shear and ball pull test results are shown. Ball shear testing measures the solder joint integrity in a plane parallel to the substrate whereas ball pull testing measures solder joint integrity in a plane perpendicular to the substrate. Both modes of testing have significance for the integrity of the joint. Figure 5 depicts ball shear testing while Figure 6 depicts ball pull testing.
Ball shear testing and ball pull testing was performed on standard ENIG deposits as well as improved ENIG and improved ENIG with additional electroless gold plated to 0.5µm gold thickness using 0.76 mm Sn/Pb solder balls, Type R flux, and a maximum reflow temperature of 230°C for 40 seconds. The improved ENIG process uses the improved pre-treatment processes as well as an improved non-cyanide immersion gold. Improved ball shear strengths are seen with the improved ENIG process. Conventional ENIG showed some intermetallic joint failures when performing ball pull testing while the improved ENIG resulted in either substrate break or solder break.
Wire Bond Testing
Gold wire bond testing was performed on the improved ENIG process with 4 microns of nickel and varying gold thicknesses from 0.04 microns (using immersion gold only) to 0.1, 0.2, and 0.4 microns (using electroless gold). Wire bond test parameters are shown in Figure 9. Results for all tests showed excellent results even for the 0.04 micron with no wire breaks at the ball or stitch bond.
Alternatives - Acid Zincate and