The shift from evaporation to sputter deposition for metalization, combined with the use of cluster tools, is yielding great benefits to forward-thinking device manufacturers, writes Andy Bavin.

Over the past several years the maturing compound semiconductor market has seen a distinct shift toward high-productivity IC manufacturing techniques. Increasing substrate diameters and higher product volumes create the desire to use the type of mass-production equipment first employed by the silicon semiconductor industry. Although partially driven by the high manufacturing capacities required throughout the boom year of 2000, this shift continues today as forward-thinking device manufacturers reap the rewards offered by the low cost of ownership of silicon-style tools. The metalization process is one of many that have shown clear advantages from this change in manufacturing philosophy. Metal deposition has traditionally been performed by e-beam or thermal evaporation techniques in batch-style tools, but single-wafer sputtering is becoming increasingly popular in advanced manufacturing facilities. This has only been made possible through the careful adaptation and development of the sputtering process for compound semiconductor applications. In this article we examine the development of sputtering for many III-V metalization processes, including lift-off applications, and look at the benefits obtained through the use of sputtering in contrast to traditional methods. Cluster tools Although modern cluster tools often have high initial capital costs, they still offer significant financial savings compared with older batch-style tools. Cassette-to-cassette processing removes the need for the large amount of time and space traditionally used to load batch tools. This reduces the amount of engineering support required by the tool, resulting in a cut in operating costs. By removing the need for manual wafer loading and physical wafer clamping, cluster tools also reduce the risk of damaging the expensive III-V substrates. Batch-style tools provide high throughputs by processing a large number of wafers at the same time. However, as substrate diameters increase, fewer can be loaded into the tool, so the overall throughput reduces. Cluster tools can provide higher throughputs when processing larger wafers, enabling one tool to replace many. They also feature three levels of vacuum integrity through the vacuum cassettes, wafer handler and ultrahigh vacuum process modules. This buffered approach to vacuum integrity and the control offered by single wafer processing ensures that process contamination is near zero, dramatically increasing the yield of the finished devices. Benefits of sputtering Although it was the productivity benefits of cluster tools that first interested metalization engineers in changing from batch evaporation to sputtering, it was soon discovered that the sputtering technique itself offered significant advantages to compound semiconductor device manufacture. The high ion energies associated with sputter deposition produce highly dense metal films. This has been proven to improve the yield of finished devices through increased resistance to contamination and better film adhesion. Further yield benefits are gained through reduced particulate contamination. Even when running at exceptionally high deposition rates the sputter target will never reach temperatures above 300 C. This prevents the target material from melting and producing the particulate "splashes" commonly associated with evaporation techniques. Unlike evaporation, where materials have to be co-deposited from different sources with different deposition rates, sputtering allows the deposition of mixed films from a single target. This ensures that the deposited film has a uniform, repeatable film stoichiometry even when performing reactive depositions in a nitrogen or oxygen process ambient. Sputtered metals In spite of all the benefits mentioned, sputtering has taken a long time to become a commercially attractive alternative to batch evaporation. Although sputtering is commonly used to blanket-deposit front and back-side pre-plate seeds, it has traditionally been difficult to use for the wider range of lift-off applications. For the lift-off process to be successful the metal deposition has to be directional to prevent resist encapsulation, and cold to prevent resist reticulation. Unlike evaporation, sputtering is neither directional or cold, so it has taken significant hardware and process development to produce a reliable sputter lift-off. Hardware developments The silicon industry initially identified the need for directional sputter deposition when depositing barrier materials into high aspect ratio contact features. This was achieved by increasing the throw distance between the target and the wafer (). Any material sputtered from the target at a low angle cannot reach the wafer, instead sticking to a shield on the inside of the process chamber. Although this technique reduces the utilization of the sputter target it does not dramatically increase the cost of the deposition, as approximately 80% of the material deposited onto the shields can be reclaimed and reused. The long-throw deposition process has a number of advantages over competing collimating techniques. Firstly it requires no additional chamber furniture, so the number of possible particle sources within the module is minimized. Secondly, because the plasma is removed from the wafer the temperature of the deposition is reduced significantly. Data obtained in Trikon s applications laboratory have shown a 20% reduction in wafer temperature when the same TaN film was deposited with a longer throw distance. Lastly long-throw is an exceptionally simple solution, enabling high equipment uptimes and reliability and further reducing the cluster tools cost of ownership. The temperature of the sputtering process can be controlled by clamping the substrate electrostatically to a cooled chuck (). Argon, passed between the wafer and the chuck, improves the thermal conductivity of the substrate, allowing the heat generated by the deposition process to pass into the chuck without substantially heating the wafer. Experiments performed in Trikon s applications laboratory show that the substrate temperature can be kept below 80 C even when depositing heavy materials such as gold at high deposition rates. The cold electrostatic chuck (ESC) therefore enables sputtered lift-off processes to be performed with exceptionally high throughputs. Process developments When depositing at high rates there is a risk of resputtering material from the substrate back onto the sidewall of the open resist window. Loosely attached metal wings can form at the edges of the lift-off feature that can delaminate under physical shock and short-circuit the device. In the worst case, resputtered material completely covers the sidewall. This prevents lift-off by stopping the chemical solvent from reaching the photoresist. A proprietary process that reduces resputtering has been developed. This enables high-throughput lift-off depositions to be performed with only minimal wing formation, as shown in . In addition to this a recent project has examined the combined effects of the resist profile, deposition process and post-deposition cleaning (Butler et al.). Optimizing the resist profile and sputter conditions, followed by a dry CO2 treatment after resist removal, produced well defined clean metal patterns (). Sputter lift-off in production Compound semiconductor device manufacturers have been quick to take advantage of sputter lift-off processes in a variety of IC fabrication steps. Sputtering offers a number of benefits to thin-film components, most importantly the repeatable quality and stoichiometry of deposited materials such as tantalum nitride (TaN), nickel chromium (NiCr) and silicon nitride (Si3N4). Moreover, as these materials are relatively inexpensive and the components consist of very thin films (less than 1000 thick), ultra-long throw distances can be used to provide exceptional deposition directionality without large increases in device costs or reductions in throughput. Lift-off has traditionally been used to pattern the various metals that form an IC s ohmic contact such as Ti, Pt and Au. In a sputter cluster tool these metals are deposited in individual process chambers, increasing the material that can be reclaimed from the chamber shielding and dramatically reducing the cost of device manufacture. The completed contacts are of exceptionally high quality and production data have shown them to have less contact resistance and increased defense against contamination when compared with evaporated equivalents. This brings increased benefits to the finished device in terms of operational powers and extended reliability. Trikon s Sigma sputtering tool offers the ability to deposit on both the front and backside of device wafers without changing any hardware. In this way a tool configured for contact metalization can also be used to deposit pre-plate seeds into through-wafer vias. The directionality of the long-throw deposition increases the proportion of sputtered metal that reaches the bottom of the via, enabling continuous coverage with reduced film thickness. The cold ESC is able to clamp through an insulating carrier into the wafer itself, providing excellent wafer cooling without carrier metalization. Today s sputter cluster tools perform numerous metalization steps in production processes worldwide. Customers for these tools have reported all of the efficiency benefits first seen in the silicon industry: higher productivity, increased throughput, reduced labor requirements and fewer wafer breakages. Interestingly, they have also noticed improvements to the performance of completed devices. Increases in yield, reductions in gate leakage and lower contact resistances have all been attributed to lift-off metalization processes using sputtering. Further reading D Butler et al. (in press) High Performance, High Productivity Interconnect by Sputtered Metal and Dry Carbon Dioxide Lift-off Techniques CS-MAX 2001 conference proceedings.