HF-Based Solutions

In HF-Based Solutions. The overall reaction involving the dissolution of silicon oxide in HF solution can be expressed as 

The elemental reactions are, however, rather complicated and remain the subject of debate. For many studies on the kinetics of silicon oxide etching, identifying the fluoride species in the solution responsible for the etching reactions has been a major concern. This is because HF is a weak acid, which ionizes only partially to fluoride ions, and its aqueous solutions may contain a significant quantity of fluorine in the form of un-ionized HF and partially ionized bifluoride HFJ. 

In an earlier study, Judge found that in solutions of NH4F and NH4CI close to neutral pH, essentially all fluoride is present in the unprotonated state and the solutions exhibit a near zero rate of Si02 dissolution. Judge thus suggested that the free fluoride ion is not one of the species responsible for the dissolution. This conclusion was further supported by Kikuyama et who found that the etch rate of Si02 in KF solutions, in which all the fluorine is in the form of free fluoride ions, is very small as shown in Fig. 4.33. The etch rate in IM KF is 0.001 A/s, which is threefold smaller than that in a HF solution of the same concentration. 

FIGURE 4.33, Relationship between etch rate and F ion concentration of KF solution. Etch rate and F ion concentration can be expressed as = 0.067 x [F] . After Kikuyama et al (Reproduced by permission of The Electrochemical Society, Inc.) 

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In particular, the hillocks formed on the (100) surface are crystallographic structures bounded by four (111) planes resulting from the anisotropic etching and their formation directly contributes to the roughness of the surfaces. Hillocks may, under certain conditions, form in HF-based solutions, " but most commonly in alkaline etchants. ° Because the (111) surface etches very slowly, the formation of hillocks leads to a decreased etch rate. ... 

Etch rate reduction is observed on highly boron doped silicon in all major alkaline etching solutions [33, 80,103]. In NH4OH etch rate reduction is stronger than in EDP and KOH. Much larger etch rate reduction, as high as 8000 times, has been observed [86], Etch rate reduction is not observed for highly doped silicon in HF-based solutions [3],... 

In HF-based solutions the ratios are close to 1, varying only slightly with different solution compositions. An exception is in NH4F solution with the addition of copper ions in which the etch rate ratio of (100)/(111) is found to be as high as 15 [65]. Addition of silver or gold ions does not have such an effect. The special effect of copper ions on anisotropicity may indicate that the copper deposition... 

With PSi templates, pore diameters from <5 nm up to >1 pm are accessible. Typical examples of SEM micrographs of PSi templates, mainly ordered macropores (5 pm), medium-sized pores (120 nm), and mesopores (20 nm), are presented in Fig. 2. The templates were prepared by electrochemical dissolution of Si in HF-based solutions (see handbook chapter Porous Silicon Formation by Anodization ). For ordered macropores (Kobayashi et al. 2006), -type Si(lOO), 10-20 Qcm, was used. Ordered etch pits were initially created on Si surface by photolithographic... 

Yoneda N, Chen S.-Q, Hatakeyama T, Hara S, Fukuhara T (1994) Selective electrochemical formyl hydrogen-exchange fluorinaticui of aliphatic aldehydes to prepare acyl fluorides using HF-base solutions. Chem Lett 849-850... 

All standard cleaning processes for silicon wafers are performed in water-based solutions, with the exception of acetone or (isopropyl alcohol, IPA) treatments, which are mainly used to remove resist or other organic contaminants. The most common cleaning procedure for silicon wafers in electronic device manufacturing is the deionized (DI) water rinse. This and other common cleaning solutions for silicon, such as the SCI, the SC2 [Kel], the SPM [Ko7] and the HF dip do remove silicon from the wafer surface, but at very low rates. The etch rate of a cleaning solution is usually well below 1 nm min-1. 

It is known that HF-HN03-based solutions etch highly doped substrates faster by a factor of about three compared to moderately doped ones. A higher selectivity is reported on addition of chemicals that reduce the HNOz concentration, like H202 or NaN3 [Mul], However, this report suffers from the fact that the etch rate was measured for separate wafers of a homogeneous doping density. For pp+ or nn+ structures, which are not spatially separated, only a low selectivity is observed, because of the autocatalytic behavior of the etchant. 

Introduction of Fluorine with Anhydrous Hydrogen Fluoride, Aqueous Solutions of Hydrogen Fluoride, and HF-Base... 

Partially functionalized cyclopolysilanes recently attracted attention as model substances for siloxene and luminescent silicon. The yellow luminescent silicon is formed by the anodic oxidation of elemental silicon in HF-containing solutions and may be used for the development of silicon-based materials for light-emitting structures which could be integrated into optoelectronic devices77. Because the visible photoluminescence of... 

Nevertheless, the acidity measurements based on the use of the benzhydryl cation indicator family87 show that the upper acidity limit of the HF-SbF5 system is reached with 10 mol% of SbF5. The weakest indicator available is protonated 4,4 -dimethoxy-benzhydryl cation (pA BH+ —23) and could not be further protonated, even in the most concentrated HF-SbF5 solutions. 

The operational performance of an HF-based ESTM technique was first described by Liu et al.71 A 15-cm piece of HF was filled with an acceptor buffer solution, after which the F1F was made into a loop. This loop-like F1F device was soaked in n-undecane, and then immersed in 1 L of a river or leachate water sample for extraction of freely dissolved chlorophenols. This FIF-loop device was also employed for selective ESTM sampling of freely available Cu+2 in leachate water.78 The selectivity stemmed from a selective liquid membrane (di-n-dihexyl ether) containing a carrier (crown ether/oleic acid) and a selective stripping agent in the acceptor solution. 

This is prepared by passing fluorine rapidly through 2% sodium hydroxide solution, by electrolysis of aqueous HF-KF solutions, or by the action of F2 on moist KF. It is a pale yellow poisonous gas (bp 145°C). It is relatively unreactive and can be mixed with H2, CH4, or CO without reaction, although sparking causes violent explosion. Mixtures of OF2 with Cl2, Br2, or I2 explode at room temperature. It is fairly readily hydrolyzed by base ... 

D. A. McCauley and A. P. Lien, ibid. 74, 6246 (1952), have measured the base strengths of xylenes in concentrated HF BF3 solutions (see also the discussion on page 578). 

Consideration of Chemical versus Electrochemical Reaction. Based on a study of the effect of pH on the relative contribution of the chemical versus electrochemical reactions in HF-NH4F solutions, Allongue proposed a reaction scheme... 

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