Sulfuric Acid-hydrogen Peroxide Mixture Research Articles

Wet cleaning of Ta-based extreme ultraviolet photomasks at room temperature

Owing to the wavelength-dependent limits of the deep ultraviolet exposure process, the semiconductor industry introduced extreme ultraviolet (EUV) lithography operating at a 13.5 nm wavelength. Traditional photomasks employ pellicles for protection; however, EUV-specific pellicles are not widely applicable to commercial processes, requiring the development of a EUV photomask cleaning method. In this study, a wet cleaning method for Ta-based EUV photomasks at room temperature was systematically examined in terms of key parameters, including the pattern step height, surface topography, and particulate count, via atomic force microscopy and x-ray reflectivity. Post sulfuric acid-hydrogen peroxide mixture (SPM) treatment, the photomask exhibited a stable step height, indicating minimal pattern degradation. Additionally, discernible alterations in the surface roughness and a decrease in particle count were observed, further indicating to the effectiveness of SPM-mediated cleaning. Conversely, following standard clean-1 (SC-1) treatment, while the pattern step height remained relatively unchanged, a notable increase in surface irregularities and macroscopic particulates was observed, suggesting a suboptimal cleaning efficiency of the SC-1 solution despite its potential for pattern structure preservation. Our room temperature wet cleaning method efficiently reduces wear-out and successfully eliminates contaminants, potentially prolonging the EUV photomask’s productivity and durability.

Nanometric Particles Removal during Photoresist Stripping

Microelectronics devices dimension have followed the Moore’s law in the last decades. In order to keep a high yield during integrated circuits production, a continuous improvement has been carried out in the particles cleaning area, switching from brushes and acoustic cleans to high velocity sprays. With the device nodes shrink, tinier and tinier particles are to be removed. Even though their adhesion and removal are well understood and documented for years [1], our industry still misses robust solutions to clean nanoparticles, keeping finest features integrity, and extremely low material consumption [2].Well chosen resist stripping conditions enable an excellent particles removal without materials consumption nor damaging 14nm node polysilicon gates. The cleaning mechanism is investigated and key parameters involving the photoresist and wet stripping will be discussed. The aim of this work is to transfer this knowledge into an industrial optimized sequence. Previous studies deal with the fluids viscoelasticity role on the particles removal from a substrate [3]. It is thought non newtonian properties are involved during the resist removal process, with the fluid generated on the wafer through the well-known interface gel formation.This paper will present results obtained through the use of 300 mm wafers intentionally contaminated with 45nm silica particles, then coated by different polymers, stripped thanks to various methods. The particles counting before and after the complete resist removal step is assigned to a KLA Surfscan SP3 blue laser diffraction tool.The stripping conditions are a key parameter to enable an excellent defects removal. Indeed the PRE (Particles Removal Efficiency) varies from a few % to 70% when switching from SPM (Sulfuric acid / hydrogen Peroxide Mixture) conditions: too chemically active (radicals rich) with a stripping rate of few hundreds of nanometers per seconds on a single wafer platform to mild conditions, requiring few minutes to remove the resist on a batch spray tool, enabling the resist transition into a gel like phase (figure 1). Two samples have also been used as reference: first a resist removal by plasma ashing that doesn’t remove any particle, proving the particles removal doesn’t occur during the resist coating step and another one without resist, with the wet stripping to quantify the PRE brought by the SC1 (Standard Clean 1). Further tests are done with various solvents to substitute the SPM, but keeping the gel phase, required for the particles lift off.Not only results on blanket but also challenging patterned wafers will be presented, showing absence of fine feature damage and PRE data, with a fair comparison with high velocity spray methods. Eventually a basic mechanism will tentatively be proposed. Moreover, such results tend to prove an interest in continuing using resist stripping in wet instead of plasma ashing. Figure 1

Development of Areal Capacity of Si-O-C Composites as Anode for Lithium Secondary Batteries Using 3D-Structured Carbon Paper as a Current Collector

To enlarge the areal capacity of Si-O-C composites as an anode for high energy Li secondary batteries, 3D-structured carbon paper is used as a current collector for electrodeposition of Si-O-C composites. For higher surface affinity between organic solvents with carbon paper, surface treatment of carbon paper is carried out using a sulfuric acid-hydrogen peroxide mixture (SPM) solution. The higher deposited Si amounts and areal capacity are obtained by SPM treatment of carbon paper due to its enhanced surface affinity between carbon paper and electrolytes during Si-O-C electrodeposition. In addition, two kinds of organic solvents, propylene carbonate (PC) and ethylene carbonate/diethyl carbonate (EC/DEC), are employed to investigate their suitability for the electrodeposition of Si-O-C composites. As a result, it is confirmed that the Si-O-C composites synthesized by EC/DEC solvents show more stable cycle abilities than in the case of using PC solvents at a high charge current density of 1.0 mA cm−2. This is due to the exfoliation of carbon paper by PC, resulting in fast capacity fading during charge/discharge cycle. Nevertheless, it is confirmed that the use of carbon paper as a substrate for Si-O-C electrodeposition is an effective way to increase deposited Si amounts and discharge areal capacity.

Infrared Study of Room Temperature Atomic Layer Deposition of SnO2 Using Sn(CH3)4 and Plasma Excited Humidified Argon

Tin oxide (SnO2) is an oxide semiconductor material with an excellent optical transparency, where it is used as transparent conducting films. It is also used as a channel material for gas sensors. Recently atomic layer deposition (ALD) of SnO2 has been studied intensively since ALD allows the conformal deposition of the oxide film with an atomic level precision. However, the high process temperature in excess of 100 °C in the conventional SnO2 ALD is a major problem for its application for flexible electronics. We developed a room temperature deposition technique with a precursor of tetramethyl tin (Sn(CH3)4, TMT) and plasma excited humidified argon. The surface reaction of the precursor adsorption and its oxidization were monitored by infrared absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry.A p-type Si(100) substrate with a resistivity of 8-12 Ωcm was used as a sample. Prior to the experiment, the sample was cleaned with buffered HF acid and sulfuric acid-hydrogen peroxide mixture solutions. The Sn precursor of TMT was introduced with a variable leak valve. A plasma excited humidified argon source was installed to the ALD chamber as shown in Fig. 1. The source gas for OH-radical is a mixture of H2O vapor and Ar made by a water bubbler. The plasma was generated in a glass tube with an induction coil with a frequency of 13.56MHz and a RF power of 30 W. Before the ALD process, the substrate surface was treated with the plasma excited humidified argon for 5 min to be terminated with Si-OH. For the SnO2 deposition, we repeated the cycle of TMT saturation and plasma excited humidified argon at RT. The condition of TMT saturation and oxidization were determined by the MIR-IRAS experiment. In this experiment, the TMT saturation exposure and oxidization time were 2×105 Langmuir and 10 min, respectively. The oxidation state of the SnO2 film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry.In Fig. 2, we show Sn 3d XPS spectra obtained from the RT grown SnO2 film, where we confirm the evolution of full oxidized state peak according to the increase in the ALD cycle number. We calculated the growth per cycle as 0.02 nm/cycle. In the conference, we present the results of RT ALD of SnO2 and discuss the surface-reaction mechanism based on the IRAS observation. Acknowledgement; This work was partly supported by JST-CREST, JSPS KAKENHI Grant Numbers 15H03536 and 15K13299. Figure 1

RT Atomic Layer Deposition of ZrO2 By Using Plasma Excited Water Vapor

Size of semiconductor devices in large-scale integration has reached the submicron range where gate oxide films in metal oxide semiconductor (MOS) devices need to be fabricated at a nanometer scale. Atomic layer deposition (ALD) is a technology for depositing dielectric films with monolayer precision by repeating adsorption of a source gas on substrate that is subsequently reactivated for further adsorption [1]. Zirconium oxide (ZrO2) is believed to be a candidate as the high-k gate oxide material [2]. The growth temperature for the ZrO2 deposition has been desired to be decreased to near room temperature (RT) because the interfacial layer is formed by the solid phase reaction with heating process [3]. In this work, we developed a RT atomic layer deposition of ZrO2 with tetrakis (ethylmethylamino) zirconium (TEMAZ) and a remote-plasma excited water vapor.A p-type Si(100) substrate with a resistivity of 8-12 Ωcm was used as a sample. Prior to the experiment, the sample was cleaned with a buffered HF acid solution and sulfuric acid-hydrogen peroxide mixture. TEMAZ as the zirconium precursor was introduced with a variable leak valve. An induction plasma system generating oxidizing gas was installed to the ALD chamber as shown in Fig.1. The source gas for the oxidizing gas is a mixture of H2O vapor and Ar gas. The plasma was generated in a glass tube with an induction coil with a frequency of 13.56MHz and a RF power of 30 W. Before the ALD process, the substrate surface was treated with the plasma excited water vapor for 5 min to be terminated with OH groups. For the ZrO2 deposition, we repeated the cycle of TEMAZ saturation and the oxidizing gas treatment at RT. The TEMAZ exposure was set to 1.12×10-3 Torr×100 s. The saturation of TEMAZ on the hydroxylated surface has been confirmed by the IR absorption spectroscopy. The plasma excited oxidizing gas was injected for 150 s. The oxidization state of the ZrO2 film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry.Fig.2 shows Zr3d spectra obtained from ZrO2 on Si grown by the present ALD. According to the increase in the ALD cycles, the Zr3d5/2 peak at around 181.9 eV evolves. The peak position indicates the full oxidized ZrO2 growth on Si [4]. Fig.3 shows the ZrO2 thickness as a function of the ALD cycle. The linear increase of ZrO2 thickness with the ALD cycle indicates that the ZrO2 is grown in the ALD mode. The growth rate is estimated to be 0.14 nm/cycle. This value is comparable with those released in the conventional thermal ALD studies [5].To conclude, we developed the non-heating, RT ALD of ZrO2 with the plasma enhanced technology. We are now investigating the electrical characteristics of ZrO2 MOS on Si and Ge, which will be released elsewhere. The RT grown ZrO2 films might be also applicable for the anticorrosion coatings on plastic material. In the conference, we present not only the details of growth experiment but also discuss the growth kinetics of RT ALD.Acknowledgement This work was partly supported by JST-CREST. We would like to express the deepest appreciation to AIR-LIQUIDE for the high quality precursor of TEMAZ.

RT Atomic Layer Deposition of Al2O3 By Using Remote Plasma Excited Water Vapor

Aluminum oxide (Al2O3) is well known as an oxide coating material. Al2O3 is also used as a barrier layer preventing water moisture and oxygen for organic light emitting diodes (OLEDs) [1]. Al2O3 has been also studied as an anticorrosion coating material [2]. On the other hand, Al2O3 has been studied as a passivation layer for Si solar cells [3]. The growth temperature for the Al2O3 deposition has been desired to be decreased to room temperature (RT) because the devices mentioned above have been demonstrated to be fabricated on plastic or flexible substrates for organic electronics. In this work, we developed RT atomic layer deposition of Al2O3 with trimethylaluminum (TMA) and a remote-plasma excited water vapor. A p-type Si(100) substrate with a resistivity of 8-12 Ωcm was used as a sample. Prior to the experiment, the sample was cleaned with a buffered HF acid solution and a sulfuric acid-hydrogen peroxide mixture solution. TMA as the aluminum precursor was introduced with a mass flow controller (MFC). A remote plasma system generating oxidizing gas was installed to the ALD chamber as shown in Fig.1. The source gas for the oxidizing gas is a mixture of H2O vapor and Ar gas. The plasma was generated in a glass tube with an induction coil with a frequency of 13.56MHz and a RF power of 30 W. Before the ALD process, the substrate surface was treated with the plasma excited water vapor for 5 min to be terminated with OH groups. For the Al2O3 deposition, we repeated the cycle of TMA saturation and the oxidizing gas treatment at RT. The TMA exposure was set to 1.67×10-3 Torr×15 s. The saturation of TMA on the hydroxylated surface has been confirmed by the IR absorption spectroscopy. The plasma excited oxidizing gas was injected for 2 min. The oxidization state of the Al2O3 film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry. Fig.2 shows Al2p spectra obtained from Al2O3 on Si grown by the present ALD. According to the increase in the ALD cycles, the Al2p peak at around 74.4 eV evolves. The peak position of 74.4 eV indicates the Al2O3 growth on Si [4]. Fig.3 shows the Al2O3 thicknesses as a function of the ALD cycle. The linear increase of Al2O3 thickness with the ALD cycle indicates that the Al2O3 is grown in the ALD mode. The growth rate is estimated to be 0.15 nm/cycle. This value is comparable with those released in the previous RT ALD studies [4-6]. We checked the composition of Al and O on the ALD grown Al2O3 film with XPS. The ratio of Al and O was calculated to be 1:1.83. The refractive index of our Al2O3 film was measured to be 1.56 at 633 nm that is the same value as that of RT grown Al2O3 reported elsewhere [7]. To conclude, we developed the non-heating, RT ALD of Al2O3 with the remote plasma source. We are now investigating the gas barrier property. The RT grown Al2O3 films might be applicable for the barrier layer of water moisture and oxygen on flexible and soft materials.

Room-Temperature Atomic Layer Deposition of HfO2 By Using Remote Plasma Source

The sizes of integrated semiconductor devices such as metal oxide semiconductor (MOS), have reached to submicron range where the gate oxide films are needed to fabricate at a nanometer scale. Atomic layer deposition (ALD) is a potential technique for depositing dielectric films with monolayer precision by repeating adsorption of a source gas on a substrate that is subsequently reactivated for further adsorption [1]. HfO2 is believed to be a candidate as the high-k gate oxide material [2]. The growth temperature for the HfO2 deposition has been desired to be decreased to near room temperature because the interfacial layer is formed by the solid phase reaction with heating processes [3]. We developed a room-temperature (RT) atomic layer deposition technique for HfO2 deposition by using tetrakis(ethylmethylamino)-hafnium (TEMAH) as a precursor and a remote-plasma excited oxidizing gas.A p-type Si(100) substrate with a resistivity of 8-12 Ωcm was used as a sample. Prior to the experiment, the sample was cleaned with buffered HF acid solution and sulfuric acid-hydrogen peroxide mixture solution. As a hafnium source, TEMAH precursor was introduced in a variable leak valve. A remote-plasma system generating excited oxidizing gas was installed to the ALD chamber as shown in Fig.1. The source gas for the oxidizing gas is a mixture of H2O vapor and a carrier gas. The plasma was generated in a glass tube with an induction coil with a frequency of 13.56MHz and a RF power of 30 W. Before the ALD process, the substrate surface was treated with the plasma excited oxidizing gas for 5 min to be terminated with OH groups. For the HfO2 deposition, we repeated the cycle of TEMAH saturation and the oxidizing gas treatment at RT. The TEMAH exposure was set to 1.67×10-3 Torr×60 s. The saturation of TEMAH on the hydroxylated Si surface has been confirmed by IR absorption spectroscopy. The plasma excited oxidizing gas was injected for 2 min. The oxidization state of HfO2 film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry. Fig.2 shows XPS spectra for Hf4f of HfO2 film grown on Si by the present ALD technique. The peak position of 16eV indicates the growth of HfO2 on Si. It is found that with the increase in the ALD cycles, the intensity of Hf 4f7/2 peak increases. We also checked the composition of HfO2 using XPS analysis. The ratio of Hf and O was calculated to be 1:2.06. The small amount of N as the impurity was detected as an impurity in the film (data not shown). We are now evaluating the concentration of the residual carbon in the film.Fig.3 shows the HfO2 thicknesses as a function of the ALD cycle. The linear increase in the HfO2 thickness with the ALD cycles indicates that the HfO2 is grown on Si with a constant rate in the ALD mode. The growth rate is estimated to be 0.26 nm/cycle. This value is comparable with those reported in previous ALD studies [4].To conclude, we successfully developed a non-heating ALD technique for HfO2 deposition at room temperature using a remote-plasma source. We are now investigating the electrical characteristics of HfO2 grown on Si and Ge as a MOS device. Moreover, the RT grown HfO2 films might be applicable in acid proof coatings on plastic materials. Detail will be presented at the conference.

Removal of UV Cured Resin Using Hybrid Cleaning Method

Ultraviolet based nanoimprint lithography (UV-NIL) technology is widely used in nanosized fine pattern transfer. NIL, which uses low pressure and low temperature, makes it possible to fabricate 3-dimensional pattern [. So, UV-NIL is one of the techniques with great potential as a new manufacturing process. But, UV-NIL process uses an expensive quartz substrate for transmission of UV light. Therefore, quartz substrate needs to be recycled to reduce the manufacturing cost. Usually, UV-NIL uses UV curable resins, whose chemical bonding and structure could be altered during UV light exposure which would result in crosslinking between the polymers. This UV cured resin with cross-linked structure is very hard to remove from the quartz substrate [. Conventionally, UV cured resin is removed by treating with sulfuric acid-hydrogen peroxide mixture (SPM) followed by ammonium hydroxide-hydrogen peroxide mixture (APM). One of the major drawbacks in using SPM-based treatment is the chemical haze formation and particle contamination on the quartz substrate [. Thus, an alternative cleaning composition will be of interest.

Effect of Pretreatment of High-Dose Implanted Resists by Activated Hydrogen Peroxide Chemical Systems for Their Effective Removal by Conventional Sulfuric–Peroxide Mixtures

Stripping of photoresists (PRs) exposed to high-dose (>;1E15 atoms/cm2)beams is one of the most challenging steps in front-end-of-line (FEOL) processing. This is due to a refractory crust that forms on the resist surface during ion implantation. The objective of this paper is to investigate the use of hydrogen peroxide systems activated by metal ion or ultraviolet (UV) light for disrupting crust formed on deep UV resist to enable complete removal of crust as well as underlying PR. Systematic investigation of variables such as hydrogen peroxide and metal ion concentration and UV intensity has led to the development of an optimal formulation for attacking the crust. Optical microscopy, confocal microscopy, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy have been used to monitor the removal of the resist. A two-step process involving pretreatment with activated hydrogen peroxide solution followed by treatment with sulfuric acid-hydrogen peroxide mixture to remove crust and underlying resist has been developed.

Stripping of Ion Implanted Photoresist by CO<sub>2</sub> Cryogenic Pre-Treatment Followed by Wet Cleaning

In FEOL processing, doping of active areas like source, drain, and extensions (NMOS and PMOS) is done by ion implantation. Un-doped regions are covered with photoresist to protect them from implantation. Ion implantation modifies the surface of the photoresist to generate a dehydrogenated amorphous carbon layer, the crust [1]. When the implant conditions are more aggressive (higher implant energy and implant dose), the hard crust becomes more and more challenging to be removed [2]. Conventionally, a plasma ashing process followed by a wet cleaning, typically SPM (Sulfuric acid/Hydrogen peroxide mixture) chemistry, can remove the implanted photoresist, but usually leads to damage and strong oxidation of the underlying semiconductor material and hence result in material or dopant loss. As the technology node migrates beyond 45nm, the photoresist removal process should also be compatible with novel materials such as high-k dielectric and metal-gate used in advanced gate stack integration. For these reasons, it is desirable to eliminate the plasma ash and SPM clean chemistry. Wet only PR removal process is studied using new chemistries like solvents that are compatible with the other FEOL process steps, however, the photoresist removal using solvents only still showed lower removal efficiency than conventional processes. It has been demonstrated that the CO2 cryogenic pre-treatment can improve the ion implanted photoresist stripping efficiency of the wet cleaning processes [3], and can also enhance the photoresist removal efficiency by the solvents.

Si cleaning method without surface morphology change by cyanide solutions

Hydrogen cyanide (HCN) aqueous solutions can remove copper contaminants from Si surfaces more effectively than hydrochloric acid/hydrogen peroxide mixture (HPM) and sulfuric acid/hydrogen peroxide mixture (SPM). When pH of the HCN solutions is adjusted at 9, Si surface morphology is not changed, while when pH exceeds 10, the Si surfaces are considerably roughed. AFM measurements show that Cu contaminants are present in the form of particles on the bare Si surfaces. XPS measurements show that the particles consist of metallic Cu. The Cu particle height decreases almost linearly with the cleaning time, and the Cu surface concentration decreases exponentially with it. It is concluded that Cu particles gradually dissolve into the HCN aqueous solutions by the direct reaction with cyanide ions at the surface of the Cu particles.

Plasma-Less Photoresist Stripping

In this paper, a novel technology for stripping implanted resists that uses wafer baking, followed by a combination of high temperature SPM (sulfuric acid / hydrogen peroxide mixture) and ultrasonics through H2SO4 will be proposed. We found that plasma-less stripping of highly dosed (1E16) resist is possible without damage in a short time, by making optimum reactive combinations of thermal decomposition and oxidation of resist (by baking and high temperature SPM), and then by removing residue using H2SO4 and ultrasonic waves.

Development of the Cleaning Method Using High-Speed Shear Flow of Ultrapure Water (2nd Report)

We notified that Cu concentration on the Si substrate surface could be reduced to less than 2.4×109atoms/cm2 by the cleaning method using high-speed shear flow of ultrapure water. But the mechanism of Cu removal in this cleaning method was not clarified. In this study, we have attempted the clarification of Cu removal mechanism by the observation and analysis of the Si substrate surface after the cleaning using high-speed shear flow of ultrapure water and SPM (sulfuric acid-hydrogen peroxide mixture) cleaning. In consequence, we showed that a lot of bosses were not observed on the Si substrate surface contaminated by Cu after the cleaning using high-speed shear flow of ultrapure water though after SPM cleaning. And we supposed the reasons about removing Cu from Si substrate and not forming a lot of bosses on the Si substrate surface by Pourbaix diagram for Cu-H2O system.

Study of Cleaning Method for Si Substrate Using High-Speed Shear Flow of Ultrapure Water

Since organic contaminants, which adhere on the silicon substrate surface, have an adverse impact on the quality of the gate oxide films, it is very highly required for the current semiconductor fabrication processes to take them off the surface before producing the films. Therefore, some kinds of cleaning method for silicon substrate, such as SPM (Sulfuric acid-hydrogen Peroxide Mixture) cleaning, ozone water cleaning and ultraviolet exposure cleaning, have applied to remove organic contaminants from the substrate surface. But these cleaning methods exert a bad influence of the global environment and spend high cost for the semiconductor fabrication. On the contrary, we have developed the new cleaning method for the silicon substrate using the high-speed shear flow of only ultrapure water and composed the new cleaning system. In the result of cleaning the silicon substrata surface contaminated by dioctyl phthalate at 1×1015atoms/cm2, we demonstrated that DOP(dioctyl phthalate) concentration on the substrate reduced at the level of 1013atoms/cm2 using our cleaning system. Moreover, we showed that our cleaning method possessed oxidizing property to Si substrate surface and supposed that DOP removal mechanism from Si substrate surface and this oxidizing property were related.

Native Oxide Growth Behavior on Silicon Surface with Various Resistivity in Ultrapure Water and CuF2 Solution

ABSTRACTWe have studied native oxide growth behavior on silicon surface with various resistivity in ultra pure water (UPW), SPM (sulfuric acid-hydrogen peroxide mixture, H2SO4:H2O2 = 4:1) cleaning and UPW contaminated with CuF2 by X-Ray photoelectron spectroscopy (XPS). The results show that the native oxide growth behavior in UPW is different from that in UPW contaminated with CuF2 and that grown by SPM cleaning. Native oxide thickness grown in UPW depends on resistivity. Native oxide thickness grown during SPM cleaning has the relationship of steric hinderance effect. However, in CuF2 solution, native oxide thickness is more influenced by the redox reaction between Cu ions and silicon atoms.

Improvement of Rinsing Efficiency after Sulfuric Acid Hydrogen Peroxide Mixture (H2SO4/H2O2) by HF Addition

We report on the improvement in efficiency of deionized water rinsing after using a H2SO4/H2O2/HF mixture which produces hydrophobic Si surfaces by means of fluorine passivation, providing better sulfur removal and shorter rinsing time than the conventional H2SO4/H2O2 mixture. After removing this passivation agent by rinsing in deionized water, the surfaces are still covered with chemical oxide, which protects them from organic contamination. Furthermore this treatment does not induce surface roughening.

陽極溶出電量分析法による金属ウラン中の微量銀の定量

Trace amounts of Ag in metallic U were determined by anodic stripping coulometry. For samples containing less than 0.4 ppm Ag, Ag was separated from U by coprecipitation with thallous iodide; then the precipitate was dissolved with a sulfuric acid-hydrogen peroxide mixture. The solution was evaporated to dryness, and Ag was determined by anodic stripping coulometry. For samples containing more than 0.4 ppm Ag, it could be determined in the presence of the large amount of U without preliminary separation. In the latter case. the effect of U was suppressed by formation of citrate complex. Satisfactory results were obtained by both methods. The effects of volume and pH of the electrolyte solution were also studied. For 10 ml of the electrolyte solution, 80-min electrodeposition was advisable, where 0.1 to 2 g Ag was to be determined. For 20 ml, 3-hr electrodeposition was advisable, where 0.3 to 2 g Ag was to be determined. Variation of pH (3 to 7) of the electrolyte did not influence the dissolution curve area. (H.H.D.)