A study has been made of the rate at which the rhombohedral faces of a natural quartz crystal are etched in a concentrated ammonium bifluoride solution. The thickness of the disturbed surface layer created by an initial mechanical lapping is estimated from rate data. The etching rate as well as this thickness are found to be sensitive to natural face orientation. The changes in the surface texture of the rhombohedral faces with repeated chemical etchings are investigated. The variation in the roughness parameters with the average depth of etch shows both directional and orientation effects. The chemical attack results in the formation of stable etch figures characteristic to the orientation of the surface on which they lie and which enlarge with repeated etchings. Finally, schematic etch figures are proposed for the differently oriented rhombohedral faces.
IntroductionChemical etching of quartz plates in HF or in NH4F" HF solutions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] can constitute a convenient procedure to develop high frequency quartz resonators [8][9][10][11][12][13][14][15][16][17][18]. Hence in the past few years many experiments on cuts commonly used for stable quartz crystal resonators, such as the AT-cut [8 13], the BT-cut [12][13][14] and the SC-cut [14-18], have been carried out. Some of these experiments [12,13,15,16,[18][19][20] provided evidence for a formation of final dissolution figures depending on the crystal orientation in agreement with other works [3,[21][22][23] showing that the etch method can constitute a chemical means of determining the crystal symmetry.In terms of the structural effect previously proposed by Ernsberger [1] to explain the variation of the dissolution rate of quartz plates with the crystal orientation, the dissolution mechanism is primarily determined by the atomic arrangement at the exposed surface of a quartz specimen. Moreover, the growth of a quartz crystal results in the development of natural rhombohedral faces intimately connected with the symmetry and the structure of this crystal. Thus a detailed study of the etching of such natural faces can perhaps permit a better understanding of the structural effect in chemical etching of quartz plates. To our knowledge there is an almost complete lack of experimental work devoted to the systematic study of the formation of etch patterns on the rhombohedral faces of quartz crystals. Hence, the purpose of this paper is to report investigations of the changes with prolonged etching in the surface topography of quartz plates whose orientations are the same as those of natural faces.
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