2. Getting involved
3. Standards Committees
4. DKE

DKE German Commission for Electrical, Electronic & Information Technologies of DIN and VDE

Overview

This International Standard applies to the manufacture of synthetic quartz, lithium niobate (LN), lithium tantalate (LT), lithium tetraborate (LBO), and lanthanum gallium silicate (LGS) single crystal wafers intended for use as substrates in the manufacture of surface acoustic wave (SAW) filters and resonators. A variety of piezoelectric materials are used for surface acoustic wave (SAW) filters and resonators. Prior to the 1996 Rotterdam IEC/TC 49 meeting, wafer specifications were typically negotiated between users and suppliers. During the meeting, a proposal was announced to address wafer standardization. This standard has been prepared in order to provide industry standard technical specifications for manufacturing piezoelectric single crystal wafers to be used in surface acoustic wave devices. Clause 3 of the standard introduces the necessary terms and definitions. Additional clauses dealing with the requirements, the sampling plan, and the test methods follow. Clause 7 contains information regarding identification, labelling, packaging and delivery conditions. Extensive information regarding measurement of Curie temperature, measurement of lattice constant (Bond method), measurement of face angle by X-ray, measurement of volume resistivity and appearance inspections are provided in additional clauses. Normative Annex A includes information for piezoelectric single crystals using Euler angle description. Material properties of piezoelectric single crystals, such as piezoelectric constants, elastic constants and dielectric constants have been generally determined in terms of the rectangular axes (X,Y,Z) which have been defined in place of the crystal axes. Wafer cuts used for manufacture of SAW equipment are generally circular cuts. The description of the Euler angles represents a possibility to describe the crystal orientation of the perpendicular with respect to the wafer surface and the direction of orientation flat corresponding with the SAW wave propagation. Informative Annex B describes manufacturing processes for SAW wafers. The first clause introduces crystal growth methods. When using the Czochralski method single crystals are grown by dipping a seed crystal into the melt in a crucible. While the crystal and/or crucible are/is rotated, the seed crystal is pulled up slowly and, after it has cooled from the melt and becomes solid, is formed into a single crystal. This method is named after the Polish developer, Jan Czochralski, who used it in 1916 for the first time to manufacture single crystals from metals. Industrial mass production using this method began with single crystals from germanium (Ge) und silicon (Si). The first crystals from LN and LT were manufactured in 1965 by the Bell telephone lab and in laboratories in the former Soviet Union. While there are r.f. induction heating and resistance heating as the heating method, regarding LN, LT and LGS, single crystals are grown generally by r.f. induction heating method. The following starting materials are generally provided: pulverised Li₂Co₃ and Nb₂O₅ (Ta₂O₅) with a Li/Nb-(Li/Ta-) mole ratio of 0,93 to 0,95 are mixed and calcinated after press forming. The polycrystalline ceramic obtained from LN (LT) is placed in a crucible and melted by heating the crucible. For LGS the starting materials are obtained by mixing La₂O₃, Ga₂O₃ and SiO₂ at a stoichiometric ratio. The mixture is pressed into pellets and annealed for several hours at temperatures of above 1 200 °C. The obtained polycrystalline LGS is used in an identical manner to LN or LT. The leading end of a seed crystal cut out in the direction of the crystal axis is carefully dipped into the melt. The seed crystal is rotated in order to generate a controlled convection form in the melt and the seed crystal (and the growth crystal) is pulled slowly from of the melt. The temperature of the melt shall be controlled close to the melting point of the material so that the growth crystal demonstrates the desired necking in the early phase of growth (reduction in cross-section at the end of the crystal). ((Insert Figure B-1. DIN EN 62276.tif)). Standard mechanical processing of a wafer begins with a crystal "as grown". Processing of the wafer then generally takes place in the following order. During cutting both ends and cylindrical grinding, both ends of the pulled out crystal are cut in the intended angle in order to specify the surface orientation. The surface is subsequently ground laterally so that a cylinder is provided with a diameter that is the same size or slightly larger than the diameter of the fully processed wafer. An area is provided on the surface of the crystal for marking the SAW direction of propagation. The orientation flat (OF) which is manufactured in a later process is positioned on this area. The SAW direction of propagation is determined based on the orientation flat. For slicing the crystal, cutting with a saw blade which is diamond-coated on the exterior side, cutting with opposing blades or reciprocating wires while simultaneously using abrasive powder are used. In many cases, the sliced wafers do not satisfy the specification for flatness, warp and thickness. The lapping is performed as an additional process in advance of mirror surface polishing in order to improve this accuracy. Generally, lapping is performed using a mixture of liquid and powder inserted between the lapping plate and wafer surface. Rough lapping abrasive or high lapping speed leads to deeper damage of the layer below the surface as well as to a rougher surface. The lapping process is often performed in several steps during which corresponding grain sizes of the lapping medium are used respectively. Bevelling, rounding of the wafer circumference is intended to prevent chipping at the edge and scratching on the main surface of the wafer by fragments in manufacturing processes. A rounded edge also decreases the probability of cracking during the subsequent heat treatment. After lapping the wafers are mirror polished in order to even out the surface so that mechanical stresses and scratches are avoided. If crystal defects and processing defects as well as damage to the layer below the surface are not completely eliminated during the polishing process, the effectiveness of the SAW device can be impaired. The responsible committee is DKE/K 642 "Piezoelektrische Bauteile zur Frequenzstabilisierung und -selektion" ("Piezoelectric and dielectric devices for frequency control and selection") of the DKE (German Commission for Electrical, Electronic and Information Technologies) at DIN and VDE.

Document: references other documents

Responsible national committee

DKE/K 642 - Piezoelektrische Bauteile zur Frequenzstabilisierung und -selektion