Etching, quartz surfaces

Low temperature etching. Our data suggests that, under hydrothermal conditions the rate of pit formation is dramatically reduced, although perhaps not completely stopped, at C = Ccrjt. Etch pits on a natural, hydrothermally-etched quartz surface therefore indicate extended dissolution times, but not necessarily etching at C < Ccrit This is because the rate of etch pit formation even above Ccr t can be significant at elevated temperatures (as shown by crystal R9). However, at low temperatures, formation of etch pits when C > C would be less likely, and natural surfaces etched at low temperature should record the saturation state of the etching fluid. 

Figure 2. The effect of dissolved Si concentration on etch pit density on quartz surfaces etched a) in sealed autoclaves for 6.5 hours, b) in a flow reactor for 6.5 hours (R5), 31.5 hours (R5SK), and 25-28 hours (R9). Reproduced with permission from Ref. 16. Copyright 1986 Pergamon Press.

Based on predicted weathering and erosion rates of the region, we estimate the profile to be several million years old. Because the soil has developed in situ, the topmost grains have reacted with water for the greatest extent of time. With depth, the total "lifetime" of the particles as soil decreases. This implies that the topmost quartz surfaces should be "reactively mature" (all fines removed, deep grown-together etch pits) and the bottom-most quartz surfaces should be "reactively young" (plentiful fines, fresh surfaces). ... 

A complex acid is probably present when silica dissolves in phosphoric acid, as it does to the extent of about 5 per cent, at 260° C.6 Concentrated phosphoric acid at 100° to 200° C. etches the surface of glass, and it has been found to attack quartz at 300° C.7 Phosphoric acid at high temperatures also destroys the glaze on porcelain. Platinum is not affected unless a reducing agent is present, which may give phosphorus and a phosphide. 

In the alkali dissolution experiments of Holt and King [1955], quartz powder after etching with dilute sodium hydroxide was treated with a solution of monosilicic acid in borate buffer at pH 8, and the results indicate that monosilicic acid is absorbed on the etched quartz with a layer of silicic acids on the quartz surface. The tendency of the silicic acid to condense and polymerize is thought to be inhibited by its coordination to the quartz surface. It is interesting to speculate that the initial high dissolution rate of silica polymorphs is caused by the desorption of silicic acid. However, in comparable experiments (Bergman and Paterson... 

Also considered for future work is the application of etched quartz as a contact surface. In this process, the wafer etched with micro-features is placed in contact with part free surfaces during contact infiltration in order to impart desired surface textures. Of particular concern in this investigation are the geometrical limits of feature size and accuracy in micro-feature rephcation. The resulting 3D-printed metal parts containing micro-feature projections could be used as a durable and low cost rapid tooling for applications such as micro-fluidics. 

Such approximation is valid when the thickness of the polymeric layer is small compared to die thickness of die crystal, and the measured frequency change is small with respect to the resonant frequency of the unloaded crystal. Mass changes up to 0.05% of die crystal mass commonly meet this approximation. In die absence of molecular specificity, EQCM cannot be used for molecular-level characterization of surfaces. Electrochemical quartz crystal microbalance devices also hold promise for the task of affinity-based chemical sensing, as they allow simultaneous measurements of both tile mass and die current. The principles and capabilities of EQCM have been reviewed (67,68). The combination of EQCM widi scanning electrochemical microscopy has also been reported recently for studying die dissolution and etching of various thin films (69). The recent development of a multichannel quartz crystal microbalance (70), based on arrays of resonators, should further enhance die scope and power of EQCM. 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

A very accurate measurement of Ccrjt would allow back-calculation of the surface energy for a given crystal. Because Ccrjt is dependent on the square of Y, such a measurement could be a very sensitive method of measuring interfacial energy at dislocation outcrops. The calculated interfacial energy from our experiments is 280+ 90 mJm- for the rhombohedral face of quartz at 300°C. Parks (10) estimated 25°C value of 360 + 30 mJm is well within the experimental error of our measurement. The best way to determine the value of Ccrjt would be to measure etch pit nucleation rate on... 

Several refinements of our experiments could test these theories further. By measuring etch pit densities as well as pit dimensions on sequentially-etched crystals, nucleation rate data and pit growth data could be collected, yielding information about the rate-limiting steps and mechanisms of dissolution. In addition, since the critical concentration is extremely dependent on surface energy of the crystal-water interface (Equation 4), careful measurement of Ccrit yields a precise measurement of Y. Our data indicates an interfacial energy of 280 + 90 mjm- for Arkansas quartz at 300°C, which compares well with Parks value of 360 mJm for 25°C (10). Similar experiments on other minerals could provide essential surface energy data. 

Fig. 3.5. Silicon etch rate as measured with a quartz crystal microbalance as a function of the bias voltage applied to the silicon surface in CF4 and Ar glow discharges. The discharge intensity was not significantly influenced by the application of the negative voltage to the silicon surface...

Figure 10.14. (a), (b) Reflection photomicrographs of weakly etched surface, and (c) the structure of Brewster fringes on an r face (model by Lu and Sunagawa [17]). Zigzag (Ain (a) and (c)) and closed lamellae (B in (a) and (c)) appear alternately. L=left-handed quartz R = right-handed quartz. 

The etch cycle is performed at the end of the growth cycle and on new quartz reactors to clean the interior surface of impurities. 

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