Total contamination prevention

The semiconductor industry has adopted the use of aqueous chemicals for wafer surface preparation. The chemicals were centered on the RCA cleans developed in the 1960-1965 timeframe. These front end-of-the line (FEOL) cleans are historically carried out in an immersion fashion wherein a batch of wafers (typically 50 or 100) is processed in chemical bath(s), followed by rinse(s) in ultra-pure de-ionised water until the sequence (recipe) is complete and the batch is ready for drying. The process is done similarly in a spray tool where wafers are sprayed with a mix of chemicals, rinsed and then dried. This technique has made use of concentrated chemicals at relatively high temperatures. Using this method, particulates, metallic, and chemical (molecular) contamination absorbed onto the wafer surface has a detrimental impact on the reliability and performance of the semiconductor devices. As IC devices have continued to shrink, it has become very clear that contamination defects on wafers must be significantly lower in density and size (below 100nm) in order to meet the needs of today's and the next generation's devices1. These stringent requirements along with economic and environmental factors have dictated that a change in the process-of-record is needed in both front-end-of-line (FEOL) and back-end-of-line (BEOL) wafer processing. This change can be in the form of using one or more of the following: 1. Dilute or even ultra-dilute chemicals 2. New chemicals 3. Lower or ambient temperature wafer processing 4. Higher purity chemicals 5. Fewer cleaning steps 6. Smaller batch sizes or even a single-wafer process. This article reviews the wafer-cleaning techniques used in manufacturing ICs to reduce, eliminate or prevent contamination on the wafer surface. The emphasis here is given to the importance of advanced wafer-surface conditioning and to "a total contamination prevention" approach. A few decades ago, a standard recipe for wafer cleaning was developed. The RCA clean served its purpose well and, as a result, it proved to be the standard for process engineers in silicon and IC manufacturing. The RCA clean utilised several chemical mixes and rinses: sulphuric acid/peroxide mix (SPM) removed organic contaminants, hydrofluoric acid (HF) etched away oxides, ammonia/peroxide mix (APM) removed particles, and hydrochloric acid/peroxide mix (HPM) removed metal contaminants. A spin, spin/rinse or boiling sump IPA dryer was used for the drying step. The chemical baths were also quite concentrated: HF at 50 parts water: one part HCl; APM at about 10 parts water: two parts ammonium hydroxide: one part hydrogen peroxide; and HPM at about six parts water: one part hydrochloric acid: one part hydrogen peroxide. This method of cleaning wafer substrates was utilised almost universally until as late as 1997. Cleaning results at 0.16mm for such a system is illustrated in Fig. 1 (K-Plot). Although it is stable and provides good performance, today many fabs cannot afford such a system due to its high cost of ownership (COO). The system performs well enough for making devices at 0.35-0.5mm technology node1-3.

Inadequate cleaning This cleaning with concentrated chemicals worked well for devices with the line widths of the time, but as device feature sizes continued to shrink to the submicron feature sizes, it was becoming problematical to get good yields or faster, reliable devices. The cleaning was not adequate to remove the contaminants that interfered with device production. Other environmental and economic factors forced fabs engineers and scientists to look for better and less expensive ways for wafer-surface conditioning. To meet such stringent requirements, engineers developed an ozonated, de-ionised water cleaning process to replace sulphuric-based chemistries. By injecting ozone into de-ionised water, process engineers were able to remove the organic contaminants previously removed by sulphuric acid1-3. New trends to use more dilute APM, HPM and HFM solutions were also investigated4,5. The process system in 1997 then consisted of an ozonated water clean, dilute hydrofluoric acid, and the traditional ammonia/peroxide and hydrochloric acid/peroxide mixes and their associated rinses. The chemicals were less concentrated than in previous systems: HF at 100:1 at ambient temperature, APM at 30:2:1 at 55°C, and HPM at 50:1:1 at 50°C. Most drying was done in an IPA-based dryer. This system was adequate for devices at the 0.25-0.35 mm technology nodes2. The typical performance of such an operation is illustrated in Fig. 2. There were several advantages to this process: 1. Metallic contamination introduced by peroxide was greatly reduced 2. Costly sulphuric acid and its associated long rinse step were replaced 3. Ozonated water left behind a clean, stable and uniform oxide film. Soon thereafter, linewidths became even smaller (in the 0.09-0.13mm range). Metallic contamination had to be reduced even further and the recipe for cleaning evolved to meet the need. Chemical suppliers have tried to replace the use of a two-step clean (APM followed by HPM), but the industry has shown little enthusiasm due to the high COO and EHS (Environmental Health and Safety) logistics of using non-traditional chemicals in the fab. In the mid-1990s, a typical clean wet station was comprised of ozonated water, ammonia/peroxide mix with a dedicated rinse and a module capable of delivering dilute hydrofluoric and hydrochloric acids. Completely removing peroxide from the recipe improved the metal signature even further, resulting in devices with enhanced device lifetimes. A typical cleaning sequence for an advanced FEOL (AFEOL) is as follows: DI:O3/dSC1/Rinse/ (HF+HCl)/Rinse/Dry. Achieving better results To further reduce the footprint and get even better process results via eliminating the presence of air/liquid interface, all these steps are done in situ wherein the chemical and rinse steps are done in the same chamber while wafers are submerged under liquid all the time. The process time was also shortened and this of course resulted in further cost-of-ownership advantages and footprint savings. In addition, drying is now carried out by a Marangoni-type dryer that consumes very little IPA. Adding ozone to the rinse water after HF treatment resulted in a clean wafer surface with stable, pure and uniform oxide film (8-10 A thick) if an O-terminated surface is desired (see Oxide regrowth). If an H-terminated wafer surface is desired, as in pre-gate or pre-metal deposition cleans, the ozone injection is easily disabled to obtain the desired results4,6. This setup is the standard in many fabs throughout the world today. Particle addition has been lessened from typically 50-100 particles at 0.3mm in the early RCA cleans to 10 particles (true adders) at 0.1mm in today's configuration (see Adders chart). Metal signature is typically below the detection limits of available analytical methods today. As shown in Table 1, an iron (Fe) level, for example, of less than 5E9 atoms/cm2 can be easily obtained when using such an AFEOL cleaning method. Other metals' levels are shown as well. The same method can be easily adapted to process a smaller batch of wafers (e.g. 25 x 200mm wafers or 13 x 300mm) or even a single wafer if required6.