X-RAY TOPOGRAPHIC STUDY OF VACUUM SWEPT QUARTZ CRYSTALS

42nd Annual Frequency Control Symposium zyxwvu zyxwv - 1988 X-RAY TOPOGRAPHIC STUDY OF VACUUM SWEPT QUARTZ CRYSTALS A. ZARKA, M.T. SEBASTIAN and B. CAPELLE Laboratoire de Minkralogie et Cristallographie associ6 au CNRS Universit6s Pierre et Marie Curie (Paris 6) et Paris 7 4 place Jussieu, 75252 PARIS CEDEX 05, FRANCE. bulk wave oscillator crystals. Sweeping prior to device fabrication has been reported (6,12,13) to improve the performance of resonators and increases the radiation hardness of quartz resonators used in precision oscillators. Irradiation of the resonators fabricated from vacuum swept quartz is highly beneficial in reducing subsequent transcient and steady state frequency offsets in a radiation environment. The variations in frequency of unswept crystals are traceable to impurities and defects. Recently Hansom (14) made systematic X-ray topographicstudy of swept and unswept quartz using synchrotron radiation and reported that the basic dislocation network remains the same after sweeping. In the present paper we report the presence of an unusual X-ray topographic contrast in vacuum swept a-quartz crystals which disappears on prolonged X-ray irradiation. Summary Aluminium is invariably present in both synthetic and natural quartz crystals which exists as a substitution for silicon. The trivalent aluminium can make only three of the bonds normally made by a silicon and hence a negative charge exists on the site due to the fourth non-bonding oxygen. This charge is compensated by impurity ions which are bound electrostatically in an adjacent *-axis channel. During sweeping the compensating impurity ions which are weakly bound near the aluminium atoms move toward the cathode. This result in the collection of negative space charges near the anode side of the crystal deforming the lattice. X-ray transmission topographs recorded from vacuum swept quartz crystals show a strong dark contrast near the anode side of the crystals due to lattice strain. This strong X-ray topographic contrast disappears when the crystals are subjected to prolonged X-ray irradiation. The negative charge formed near the anode side of the crystals during sweeping is compensated by forming aluminium-hole and aluminium-hydroxyl centres during the irradiation. zyxw Experimental methods and results Y-cut synthetic crystals (15x15~1mm) were used for the sweeping experiments. The sweeping was done in vacuum at 500°C with platinum electrodes and a field of 2500 V/cm was applied parallel to thef-axis of the crystals. The electrodiffusion process was carried out for 11 to 16 days. After sweeping, the crystals were studied by X-ray Lang projection topography with MoKa radiation. It is observed that the topographs of all the swept crystals show a strong dark contrast near the anode side of the crystals. Figure 1 shows the X-ray transmission topographs (10.0 reflection) of a typical crystal which was swept for 16 days. In order to study in detail the strong contrast which appeared on the topograph near the anode side, we took (15) a series of topographs from different X-ray reflections. Surprisingly it was found that this contrast disappeared gradually after recording few topographs (after 4 or 5 days exposure to X-rays). Notice the disappearance of the dark contrast in figure 2 whichwas initially present in figure 1. It is observed that the contrast appeared in all types of reflections (types 10.0 and 00.3). Introduction Quartz crystals as a consequence of their piezoelectric properties are widely used as frequency standards and hence the importance of the use of high quality sample crystals. In both synthetic and natural quartz crystals aluminium atoms invariably get substituted for silicon. From an ionic point of view aluminium is a +3 entity in a +4 site. The trivalent aluminium can make only three of the bonds normally made by a silicon and hence a negative charge on the site due to the fourth non-bonding oxygen. This charge is compensated by an alkali ion (Li+, Na+) or a proton (H+) electrostatically bound in an adjacent $axis channel nearby or holes trapped at oxygen ions. The aluminium-hole centres are formed by the absence of an electron from a non-bonding orbital of an oxygen ion adjacent to the aluminium. Sodium is an important impurity in synthetic crystals since NaOH and Na2C03 are used as mineralisers during the hydrothermal growth of the crystals. Because of the coulombic attractive force of the interstitial ions and the holes with the aluminium ions, and because of the high mobility of both interstitial ions and the holes, these charge compensators are usually located adjacent to the substitutional aluminium ions and this gives rise (1) to either A1-OH-, Al-Li+, Al-Na+, or Al-hole centres. Sweeping or electrodiffusion is a process which selectively exchanges monovalent impurity ions in a-quartz crystals and has been studied for many years (2-7). The sweeping is reported (6,8-11) to lower the production of etch channels significantly which is very important in the production of devices by photolithographic techniques and of very high frequency zyxwvutsrqpo CH2588-2/88/0000-208 $1.00 C 1988 I E E E Discussion Several workers observed (16-21) an anomalous increase in the X-ray and Y-ray diffracted intensity of a-quartz under the influence of electrostatic fields up to 104V/cm. Charpa et al. (20) observed a change in the X-ray Bragg diffracted integrated intensity with clear hysteresis effects on slowly varying the electric field. By Y-ray diffraction experiments Dousse and Kern (19) showed that the reflectivity of (110) planes ofa-quartz can be increased by 50% by applying a high electric field. Several investigators observed (20-23) a space charge collection in swept crystals. 208 zyxwvuts zyxwvutsrqp zyxwvutsrqpon zyxwvu Alpha quartz crystals invariably contain alumi- nium (A13+) which exists as a substitution for silicon (Si4+). The effective charge so created is compensated by positive interstitial ions such as alkalis and protons. When an electric field is applied along the z-axis of the crystal in vacuum at elevated temperatures, the weakly bound alkali ions move toward the cathode. If additional mobile positive ions or charges are not available at the anode, sweeping would result in the accumulation of negative space charges near the anode. The strong dark X-ray contrast which appeared on the topographs recorded from vacuum swept crystals are due to lattice strain produced by the accumulation of negative charges. The contrast with striations parallel to the z-axis, indicating inhomogenous lattice strain, is not due to dislocations since dislocation contrast vanishes on certain reflections depending on the Burgers vector and do not disappear on irradiation. The disappearance of the contrast on prolonged irradiation is due to the neutralisation of the negative charges. It is well known (1,3,13,24-27) that quartz crystals on irradiation produces Al-hole and A1-OH- centres. The ESR data of vacuum swept crystals showed (27) that the Al-hole concentration increases by a factor of four during irradiation. J . R . Hunt and R.C. Smythe. Proc. 39 Ann. Freq. Contl. Symp. (1985) 292. J.G. Gualtieri. Proc. 39 Ann. Freq. Contl. Symp. (1985) 247. D.B. Fraser. Physical Accoustics, vol. 59. Electrodiffusion of impurity ions in alpha quartz crystals in vacuum leads to the accumulation of negative charges near the anode side of the crystals. This results in a strained crystal lattice which gives a strong X-ray topographic contrast near the anode side of the crystal. During irradiation of such crystals the negative charges created is neutralised by forming Al-hole and A1-OH- centres. W. Hansom. Proc. 41 Ann. Freq. Contl. Symp. (1987) 228. M.T. Sebastian, A. Zarka and B. Capelle. J. Appl. Cryst. 11. (1988) in press. W.J. Spencer. Physical Accoustics, vol. Academic Press New-York. 2 K. Yasuda and N. Kato. J. Appl. Cryst. (1978) 705. 11 K. Gouhara, Y. Bessho, K. Yasuda and N. Kat0 Jap. J. Appl. Phys. 11. (1981) L503. A36 E. Charpa, H. Ihringer, H. Jagodzinski and A. Kneifel. Z. Naturfor. A27 (1972) 469. M. Calamiotou, J. Psicharis, S.E. Filippakis and E. Anastassakis. J. Phys. (1987) 5641. H.E. Wenden. Amer. Miner. 9 (1957) 859. E.L. Milne and P. Gibbs. J. Appl. Phys. (1964) 2364. J.A. Weil. Radiation Effects The authors are grateful to Mr. Buisson and Mr. Bignon of SICN for their help in sweeping the crystals studied here. 21 26 (1975) 261. W.A. Sibly, J.J. Martin, M.C. Wintergill and J.D. Brown. J. Appl. Phys. 50 (1979) 5449. zyxwvutsrq zyxwvutsrqpo zyxwvutsrq zyxwvutsrq zyxwvutsrqp zyxwvutsrqpon (27) A. Kahan, F. Euler, H.G. Lipson, C.Y. Chen and L.E. Halliburton. Proc. 41 Ann. Freq. Contl. Symp. (1987) 216. References Halliburton, N. Koumvakalis, M.E. Markes and J.J. Martin. J. Appl. Phys. 52 (1981) 3565. (2) J.C. King. Bell System Techn. J. ( 3 ) A. Kats. Philips Res. Repts 17 2 (1959) 573. (1962) 133. ( 4 ) H. Jain and A.S. Nowick. J. Appl. Phys. (1981) 1477. (5) E.H. (1968) J.C. King and H.H. Saander. IEEE Trans. Nucl. Sei. NS-19 (1972) 23. Acknowledgements (1) L.E. (1968) J.C. King and D.R. Koehler. Precision Frequency Controll, Vol. 1 (1985) 147. Academic Press, New-York. J.Cl. Dousse and J. Kern. Acta Cryst. (1986) 966. Conclusion 5 Snow and P. Gibbs. J. Appl. Phys. (1964) 2368. 53 2 (6) J.J. Martin. Proc. 41 Ann. Freq. Contl. Symp. (1987) 167. (7) R.N. Brown, J.J. O'Connor and A.F. Armington. Mater. Res. Bull. E (1980) 1063. (8) G.R. Johnson and R.A. Irvine. Proc. 41 Ann. Freq. Contl. Symp. (1987) 175. (9) J.F. Balascio and A.F. Armington. 40 Ann. Freq. Contl. Symp. (1986) 70. 209 zyxwvutsrqponmlkjihgfe Fig. 1 - X-ray transmission topograph using 10.0 reflection of a crystal swept for 16 days. Fig. 2 210 - X-ray transmission topograph using 10.0 reflection of the same crystal recorded on the seventh day of irradiation by X-rays. The contrast on Fig. 1 has disappeared here.