Using Undissolved Ozone in Semiconductor Cleaning Applications
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A CANON COMMUNICATIONS LLC PUBLICATION MARCH 2004 www.micromagazine.com SSPECIALPECIAL AAPPSPPS SpraySpray coatingcoating findsfinds growinggrowing numbernumber ofof applicationsapplications inin MEMS,MEMS, otherother devicesdevices AADVDVANCEDANCED PPRROCESSOCESS CCONTROLONTROL SimpleSimple statisticalstatistical controlcontrol approachapproach improvesimproves Altis’sAltis’s defectdefect monitoringmonitoring MMAATERIALSTERIALS IINTEGRATIONNTEGRATION NationalNational optimizesoptimizes SOGSOG process, process, rreduceseduces relatedrelated costscosts byby halfhalf EEXCLUSIVEXCLUSIVE IINTERVIEWNTERVIEW AppliedApplied Materials’Materials’ MikeMike SplinterSplinter GREEN MANUFACTURING Using dissolved ozone in semiconductor cleaning applications Christiane Gottschalk, MKS Instruments, ASTeX; and Juergen Schweckendiek, ASTEC zone (O ), an allotrope of water, to disinfect swimming pools, and O 3 oxygen, is a highly reactive gaseous oxi- to prevent the growth of microorgan- dizing agent that absorbs harmful ultra- isms in cooling-tower systems. This ar- violet (UV) radiation, thus enabling life ticle surveys ozone applications in the on earth. The first ozone generator was semiconductor manufacturing indus- developed by Werner von Siemens in try and provides sample data from users and researchers. A survey of several wafer-cleaning techniques Ozone Uses in highlights the advantages of dissolved ozone over the IC Industry traditional sulfuric acid–peroxide methods and For more than 20 years, semiconductor in- RCA cleans using SC-1 and SC-2 mixtures. dustry researchers have investigated the use of ozone for wafer-cleaning Germany in 1857. In 1896, Nikola Tesla and resist-stripping applications. To obtained the first U.S. patent for an lower chemical consumption and dis- ozone generator based on electric dis- posal costs as well as to improve cleaning charge in an oxygen-containing gas, efficiency, ozone has been studied during which is the primary method of ozone the past decade as an alternative to generation used today. traditional sulfuric acid–peroxide and The number and diversity of ozone RCA cleans using basic (SC-1) and acidic applications have increased enormous- (SC-2) hydrogen peroxide mixtures. It is ly since ozone’s first full-scale use as a effective because of the multiple influ- disinfectant for drinking water in Nice, ences exerted by the disinfecting activity France, in 1906. It is widely used to treat of O3 and O3-derived oxidizing species and purify ground and surface water as such as OH radicals. well as domestic and industrial waste- In chip fabrication processes, ozone is using hydrofluoric acid (HF), enabling metal removal. DIO3 alone may be sufficient for the removal of adhered particles if they are of an organic nature. However, particles on silicon dioxide are generally removed by etching the oxide O3 GAS beneath the particle with dilute hydrofluoric acid (dHF) and avoiding particle redeposition. If the bulk of the particle is not dissolved by dHF, O3 as an oxidant can create a new layer that is etchable by HF. This is true for silicon particles PARTICLE RINSE dHF-O REMOVAL 3 and silicon surfaces. The formation of an oxide layer on silicon is a self-limit- ing process. At room temperature, the oxidation of the sili- con surface creates an oxide layer that can measure up to Figure 1: Schematic diagram of three-step ACD approximately 1 nm thick. The quality of the thin oxide layer cleaning system configuration. depends on other parameters, such as humidity. In tests in- principally used to clean wafers; eliminate organics, metals, volving spray and immersion tools, the initial oxide growth and particles; remove photoresist; and disinfect DI-water rate was a function of ozone concentration.2 In immersion facilities. Cleaning with ozone always involves oxidation. tools, final oxide thickness was dependent on the initial Process differentiation depends on the primary purpose ozone concentration and pH value, indicating a reaction- of the cleaning step. limited process.3 However, since a static system was used in Removing Organics. Much information on the ability of these tests, the decay and consumption of ozone may have ozone to remove organics has been derived from research on influenced the results. the treatment of drinking water and wastewater.1 Ozonated Several research studies have been published on cleaning DI water (DIO3) has a high oxidation potential and can de- processes that combine ozone with HF, hydrochloric acid grade organic contamination. Its removal efficiency depends (HCl), or both. In these studies, the chemicals were applied on the type of organic species, the ozone concentration, and in sequence or as mixtures in spray, immersion bath, or the reaction regime. single-wafer processes.4–9 Ozone dissolved in ultrapure water generates an OH ac- The single-wafer spin cleaning with repetitive use of tive radical during self-decomposition. While the ozone de- ozonated water and dilute hydrogen fluoride (SCROD) composes the organics directly, the active radical decom- method alternately dispenses dilute dHF and DIO3 on a poses them indirectly. The different reaction pathways lead spinning wafer.9 Depending on the desired final condition of to different oxidation products. The direct ozone reaction the surface, the process ends with either a dHF/rinse or DIO3/rinse followed by spin drying in nitrogen. A one- minute, three-cycle process can remove 87% of Al O par- DIO3 can degrade organic 2 3 ticles, 97% of Si3N4 particles, and 99.5% of polystyrene latex contamination. Its removal particles. In contrast to methods that apply dHF and DIO3 efficiency depends on the simultaneously, repetitive SCROD cleaning does not in- crease surface roughness. organic species, ozone The advanced cleaning and drying (ACD) method devel- oped by ASTEC (Berg, Germany) uses a mixture of dHF concentration, and reaction and O3, combining metal removal and drying into one pro- regime. cess. In combination with a particle-removal step using ei- ther a traditional SC-1 clean or a surfactant, the ACD pro- pathway is selective, with normally slow reaction-rate con- cess consumes up to 60% less chemicals than the classical stants. The indirect OH reaction is fast and nonselective, RCA process. The result is a hydrophobic wafer that, if nec- but it must be activated by initiators such as a high pH, hy- essary, can be reoxidized in the gaseous ozone directly above drogen peroxide, or UV radiation. Although a fast reaction the dHF/O3 bath, as shown in Figure 1. is desirable, a reaction by radicals alone should be avoided. Figure 2 presents typical metal contamination levels on In many cases, active species must act directly on a sur- <100> silicon wafers after an HF/O3 clean, a modified RCA face, since species that are generated too far away from the clean, and alkaline etch. After only one HF/O3 cycle, con- surface become deactivated and lost. tamination levels were reduced to ~1 × 109 atoms/cm2 or less Removing Metal and Particles. DIO3 alone cannot effec- for all measured metals. The metal removal/drying step can tively remove such metals as iron, nickel, aluminum, mag- be performed without changing the number of particles on nesium, and calcium, which deposit on silicon surfaces such the wafer surface and without a significant increase in the as metal hydroxides or metal oxides. Depending on their number or size of crystal-originated particles. nature, the metals may be incorporated into the oxide layer Photoresist Removal. Traditional wet chemical process- or reside at the Si-SiO2 interface. They can be removed with es used to remove photoresist rely on concentrated sulfuric acids acting as ion exchangers, or the oxide can be dissolved acid combined with hydrogen peroxide (SPM) or ozone vapor at elevated 100 × 1010 temperatures.12 HF/O3 CLEAN The addition of MODIFIED RCA CLEAN scavengers and the ALKALINE ETCH increase in temper- 10 × 1010 ature have im- proved strip rates. 2 However, photore- sist removal using 1 × 1010 a wet clean process ATOMS/cm remains a chal- lenge that depends 0.1 × 1010 on the type of resist and postexposure processing used. Disinfection. 0.01 × 1010 Ca Fe Ni Cu Zn Ti V Cr Mn Co The introduction of ozone into water METAL treatment systems about a century Figure 2: Typical residual-metal surface concentrations after three types of clean processes ago was directed at on <100> silicon wafers: a one-step process in an HF/O dryer, a modified RCA clean, and 3 the disinfection of an alkaline etch. microbiologically (SOM). An alternative process using ozone dissolved in polluted water. In the semiconductor world, ozone is DI water provides environmental benefits and lower costs. used to disinfect water purification systems. However, Photoresist-strip rates in DIO3 increase with increas- chemicals such as chlorine or chlorine dioxide, which are ing ozone concentration or temperature (at a constant used to purify drinking water, are not acceptable in the IC ozone concentration). Unfortunately, with increasing industry. An advantage of ozone is that it decays back to temperature, the saturation ozone concentration in water oxygen. However, in a closed water-purification system, decreases while the rate of ozone decay increases. The the oxygen concentration can accumulate to levels that ozone-delivery process must be carefully optimized to are higher than specified in The International Technol- achieve the maximum photoresist removal rate. ogy Roadmap for Semiconductors.15 Several attempts to use ozone in resist-strip processes An International Sematech study on high-purity water are reported in the literature. For example, ozone has disinfection reported that reduced dissolved-oxygen been mixed with hot DI water at the point of use in an ef- concentrations were achieved by combining a Gore-Tex fort to achieve a high ozone concentration, and scav- membrane contact system from W. L. Gore & Associates engers have been added to prevent ozone decay.10–13 It (Newark, DE) with a high-capacity ozone generator from has been found that strip rates are influenced by the mass ASTeX (Berlin, Germany).16 An oxygen concentration of transfer rate of dissolved ozone from the bulk liquid into ~240 ppb was obtained.