Sputtering Deposits Evaporation-Quality UBM for Flip-Chip

		At a Glance
	Evaporation is typically used to deposit the under bump metal (UBM) for flip-chip because it provides an interface-free material transition. A dual cathode source is used to sputter UBMs with comparable quality and additional benefits.

Cost and technology¹ are the driving forces in the growth of flip-chip packaging. Estimates of 38% growth per year over the next five years have been reported.² As the cost per die drops on volume production ICs, such as DRAMs, the packaging of a die is becoming more expensive than the die itself. Cost objectives are <$0.005 per lead. Direct chip attachment provides one approach to minimizing the packaging cost.

Flip-chip attachment eliminates the need for wirebonding and provides a robust metallurgical attachment. In logic applications, lead counts in peripheral die attachment is limited by wirebonding capability and accuracy. To increase I/O counts, a shift from wirebonding to area array packaging is taking place. Flip-chip attachment provides an ideal  route to forming a high lead count, small-area package, saving considerable MCM or PC board real estate. This shift is occurring for lead counts above 800.

As device performance exceeds the 200 MHz range, wirebonded assemblies suffer from signal degradation because of cross talk between adjacent wires seriously impacting signal-to-noise ratios. Flip-chip attachment allows one to densely package ICs in telecommunications, while reducing interference.

Flip-chip technology has been in production for many years. One of the oldest methods is the "C4'' process.³ Today there are several variations, but in general, the C4 metallurgy involves a chromium (Cr) underlayer, a chromium-copper (Cr-Cu) interlayer and a Cu overcoat. This top copper layer may be further electroplated with Cu, coated with solder (by electroplating, evaporation or screen printing) or coated with gold for oxidation prevention. Evaporating the Cr/Cr-Cu/Cu under bump metallurgy (UBM) is the most prevalent approach.

1. The S-Gun source has a dual cathode arrangement.

C4 is a batch process, coating more than 10 wafers at a time. Typically, the process begins with e-beam sublimation of the Cr. Once stabilized, and upon depositing the required underlayer thickness, Cu evaporation is begun and runs concurrently. Increasing the Cu deposition rate, while holding the Cr process stable, produces a phased interlayer structure. The final Cu layer is deposited by shutting off the Cr source. Evaporation provides a seamless process, eliminating any interfaces among the three layers.

However, there are several key drawbacks to evaporation. As wafer size increases, the value of a fully processed wafer is very high by the time it reaches the UBM stage of processing. Estimates of wafer value at this point are $10,000 for 200 mm wafers and $40,000-$50,000 for microprocessor-laden 300 mm wafers. The monetary risks of a UBM process failure, resulting in the loss of a complete batch, leads engineers to seek a single-wafer process. Also, evaporation process throughput is typically <12 wafers/hr for 200 mm wafers. Large process chambers with heavy platens require long pumpdown times after preventive maintenance (PM) and batch exchanges. Finally, evaporation is a low-energy process; the bonds to the substrates are mechanical or physical rather than metallurgical.

Batch or in-line sputter processes are not ideal solutions either. They are also throughput limited and suffer from similar product loss risks. Cluster systems provide an ideal solution. They can also produce the interface-free UBM structure that evaporation provides, if it is done in a single module. A phased transition from the metal adhering to the top level device metal, aluminum, to a solderable or plateable material, typically Cu, is desired for excellent mechanical properties such as bump shear and bump pull strength. Using separate sputter sources for Cr, a fixed Cr-Cu alloy and Cu would leave well-defined interfaces between the layers.

Dual cathode approach

Sputtered Films has developed a sputter source with two concentric dual ring cathodes that are independently powered.  This "S-Gun" source allows deposition to be phased from one metal to the other. After the top level device metal and overlying photoresists have been prepared using RF etching or ion milling, contact metals and alloys such as Cr, titanium (Ti), titanium-tungsten (TiW) or tantalum (Ta) can be deposited and phased to the solderable or plating seed layer.

Deposition rates for the  cathodes are linear with power when operated in the required power range. Calibrating the deposition rates of the individual cathodes for the phasing process can be done with simple automatic control algorithms. To counter the impact of the variations on cosine distributions of different materials, the source design allows engineers to position the targets at different throw lengths (target to wafer spacing), thus optimizing processes (Fig. 1).

Additional advantages are available from the use of sputtering to deposit the UBM. A biasable anode allows additional surface modification during deposition, especially for cold depositions required when photoresist is present. Sputtering chambers are available with getter pumping to keep films contamination-free.

Also available are cluster tool arrangements to maintain high wafer throughput independent of wafer size. In foundry activities, PM and size conversion are possible in <2 hrs. A cost-of-ownership <$10/wafer can be achieved for a fully utilized cluster tool, which can help foundries achieve the often noted goal of <$50/wafer for the entire flip-chip packaging process.

Films characterization

Uniformity is sufficient to develop a highly reliable metallurgical bond across a wafer. On 200 mm wafers, individual metal film nonuniformity ranged between <3 and 7% (1s) using a four-point probe resistivity mapper. These values are significantly lower for 150 mm wafers. Using profilometry, the phased film nonuniformity is <5% (1s). In bump shear tests, such uniformity has produced excellent bump shear quality as compared to the evaporative UBM (Table 1).

Auger analysis of three combinations -- Cr/Cu, TiW/Cu and Ta/Cu -- all show a well-defined phased structure with linear compositional gradients. There is no evidence of any interface contamination as might be expected from a multimodule, multilayer approach. Figure 2 illustrates a Cr/Cr-Cu/Cu Auger compositional profile.

As seen in Figure 3, SEM examination of the fracture surface of a cleaved wafer also shows the interface-free structure. The bright lines, which may be construed as an interface, are the result of film wicking during cleavage. TEM examination also shows the gradual interface-free structure. The phased region exhibits a very fine microcrystalline structure. AFM examination (Fig. 4) reveals a top Cu surface structure that is ideal for solder wetability.

Using proprietary plasma engineering, film stress is controlled to levels well within the requirements for good UBM structures. A low gas flow process has been developed for use with a modified S-Gun, which provides fully dense, bright metal films exhibiting excellent adhesion, uniformity and stress. The most critical film is the Cr. Figure 5 illustrates the relationship among process gas flow, secondary power and film stress for Cr. Films on 200 mm wafers were deposited at 4 kW. Cu stress is quite insensitive to gas flow and exhibits stress values of about 3 x 10⁹ dynes/cm² (tensile).

Bump mechanical performance

Comparisons of bump mechanical performance between users is difficult in that there is no standardization on bump diameter and solder. Two types of data have been reported by users (Table 2). The first is bump pull, and the second is a multiple bump shear test. Because the number of bumps sheared varies from tester to tester, the data is normalized to specific customer specifications. In all cases, failure occurred in the solder, not in the UBM. There was no evidence of ball lifting.

Conclusion

The single-module sputtering approach, using a dual cathode source, provides a UBM that mirrors the quality of the evaporative approach. An interface-free phased structure yields a highly adherent, mechanically sound UBM that meets all requirements for flip-chip packaging. Using this technology in a cluster tool allows for a high-throughput, single-wafer processing solution, with a through-put independent of size and a minimized loss risk.

References

1. The National Technology Roadmap for Semiconductors, Semiconductor Industry Association, 1994, p. 132.

2. B. Levine, Electronic News, 42, 2106, Cahners Publishing Co., Newton, Mass., March 4, 1996, p. 1.

3. John H. Lau, ed., Flip Chip Technologies, McGraw-Hill, New York, N.Y., 1996), p. 26.

Daniel Marx is vice president of sales at Sputtered Films. He has a bachelor's degree in chemistry from Temple University and advanced degrees in metallurgy from Pennsylvania State University.

Abdul Lateef has a master's degree in mechanical engineering with emphasis in material science from the University of Nebraska-Lincoln. He became an applications engineer at Sputtered Films in 1996.

Andrew Clarke, senior vice president, is in charge of engineering for the Endeavor PVD product line at Sputtered Films Inc.

The authors can be reached at phone (805) 963-9651 or fax (805) 963-2959.