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Page 1: SCANNING ELECTRON MICROSCOPY OF RESONATING QUARTZ CRYSTALS

SCANNING ELECTRON MICROSCOPY OF RESONATING QUARTZ CRYSTALSR. J. Gerdes and C. E. Wagner Citation: Applied Physics Letters 18, 39 (1971); doi: 10.1063/1.1653552 View online: http://dx.doi.org/10.1063/1.1653552 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/18/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Interface circuits for quartz crystal sensors in scanning probe microscopy applications Rev. Sci. Instrum. 77, 083701 (2006); 10.1063/1.2238467 Scanning probe microscopy with quartz crystal cantilever Appl. Phys. Lett. 87, 074102 (2005); 10.1063/1.2031937 Tapping mode quartz crystal resonator based scanning force microscopy Rev. Sci. Instrum. 76, 016106 (2005); 10.1063/1.1832292 High-speed near-field scanning optical microscopy with a quartz crystal resonator Rev. Sci. Instrum. 73, 2057 (2002); 10.1063/1.1470233 Scanning localized viscoelastic image using a quartz crystal resonator combined with an atomic forcemicroscopy Appl. Phys. Lett. 74, 466 (1999); 10.1063/1.123033

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Page 2: SCANNING ELECTRON MICROSCOPY OF RESONATING QUARTZ CRYSTALS

VOLUME 18, NUMBER 2 APPLIED PHYSICS LETTERS 15 JANUARY 1971

SCANNING ELECTRON MICROSCOPY OF RESONATING QUARTZ CRYSTALS

R. J. Gerdes and C. E. Wagner

Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 8 September 1970; in final form 23 November 1970)

SL- and AT-cut quartz crystal resonators which are commonly used in frequency control devices were examined in the scanning electron microscope. The quartz slices were reso­nated at frequencies ranging from 455 kHz to 15 MHz. Variations in surface charges which were produced during resonance strongly affected secondary electron emission. Therefore. micrographs could be obtained which were characteristic of the various modes of motion of the resonators. From displacements of topographical features during resonance approximate values for the amplitudes of face and thickness shear were obtained.

Ultrasonic vibrational modes in quartz crystal resonators have been studied by a variety of tech­niques because of the great importance these crys­tals have as frequency control devices in various fields of communication. The use of the probe method1•2 has led to information about surface polarization of selected quartz crystals. Light optical methods3 have provided information on sur­face displacements of resonating crystals. X-ray diffraction topography has produced images of strain variations in resonators along their various crystallographic directions. 4 The direct observa­tions of electric fields on solid surfaces is pos­sible with the scanning electron microscope (SEM).5 Spivak and co-workers6 have reported on SEM studies with germanium-coated quartz and lithium niobate resonators. The application of time-resolved SEM to bulk-effect oscillators has also been described by McDonald et al. 7

In the present work it is shown that quartz crys-

FIG. 1. AT-cut resonators excited in fundamental mode of resonance at 5 MHz.

tal resonators which are actual frequency control devices may be studied without further surface preparation in the conventional SEM.

SL- and AT-cut quartz crystals were utilized in these studies. The AT-cut crystals consisted of circular plano-convex quartz slices of t-in. diameter with an 8-in. radius of curvature for the convex side. Either annular or circular aluminum electrodes had been evaporated at the centers of the AT-cut crystals. In the case of the SL-cut crystal the dimensions were 10x4xO. 7 mm3

Thin-film electrodes covered the faces of the quartz slab completely, but not the edges. A con­ventional crystal impedance meter was used to drive the resonators at levelS ranging from 2 to apprOximately 10 mW. The emissive mode of operation in the SEM was used throughout the ex­periments. As a 2-kV electron beam was used, charging on the quartz surfaces was no problem. Contamination layers, however, which are pro-

FIG. 2. Resonator excited at 5 MHz displaying modes due to flexure and thickness shear.

Copyright@ 1971 by the American Institute of Physics 39

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Page 3: SCANNING ELECTRON MICROSCOPY OF RESONATING QUARTZ CRYSTALS

VOLUME 18, NUMBER 2 APPLIED PHYSICS LETTERS 15 JANUARY 1971

FIG. 3. SL-cut quartz resonators. (A) dormant crystal, (B) during resonance at 455 KHz.

duced through interaction of the incident electron beam with the residual gases in the microscope, increased the conductance of the quartz surfaces to such a level that the charge pattern of the reso­nating crystals disappeared after prolonged ob­servation in the SEM. The quartz crystals were therefore repeatedly cleaned in chromic acid.

Figure 1 is a scanning electron micrograph of part of a typical AT-cut resonator. The top of the picture shows a portion of the circular aluminum electrode while the lower portion shows part of the quartz crystal. The resonator was excited in its fundamental mode of resonance at 5 MHz. The curled and twisted band structure on the quartz surface changed when a different mode of

40

resonance or a different crystal was used. The appearance of the pattern in the SEM occurred only within a few kilohertz of resonance.

The bright and dark areas in the pattern indi­cate the positions of the antinodes and nodes of dis­placement. During ultrasonic excitation, the electric polarization is zero at an antinode of dis­placement. Secondary electron emission from an antinode should therefore be about the same as from a quartz crystal surface which has not been ultrasonically excited. The bright areas are there­fore the locations of the antinodes of displacement. The polarization during excitation reaches a max­imum at a node of displacement where during the first few cycles of resonance, when the polariza­tion is at its negative maximum, more electrons are emitted than at an antinode. The result should be a net deficiency of electrons, positive charging of these areas, and therefore a lower secondary electron yield at the nodes relative to the antinodes. The nodes will appear dark or black in the scanning electron micrographs. This relationship between positive charging and low secondary electron yield is consistent with observations by other authors. 8

From x-ray diffraction topography; it is known that the modes of motion observed in the AT-cut resonators are coupled modes of flexure and thick­ness shear. Motions due to flexure caused the pattern in Fig. 1.

Figure 2 shows an AT-cut resonator with an­nular electrode operated within a few kilohertz of its fundamental mode of resonance at 5 MHz. The area inside the annular electrode displayed a pattern due to a thickness-shear mode, while out­side of the electrode a flexure mode was visible. A surface defect on the quartz surface also inter-

FIG. 4. Direct observation of the amplitude of face shear; (A) dormant crystal, (B) low drive level, (C) high drive level.

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Page 4: SCANNING ELECTRON MICROSCOPY OF RESONATING QUARTZ CRYSTALS

VOLUME 18, NUMBER 2 APPLIED PHYSICS LETTERS 15 JANUARY 1971

rupted the pattern due to flexure. Sometimes such crystals showed only flexure modes over the en­tire area.

A dormant SL-cut resonator is seen in Fig. 3(A). From x-ray diffraction studies, anti nodes of vi­bration in the x direction due to face shear were to be expected in areas labeled a, with the nodes in areas b. Because of the above-mentioned thin­film coating, voltage contrast during resonance at 455 kHz was expected only along the edges. The nodes and antinodes occurred approximately where the amplitude was greatest as determined from x-ray diffraction studies [Fig. 3(B)].

The micrograph of Fig. 4(A) was taken at the edge of the crystal slab in the center of region a. Figure 4(B) and 4(C) show the same area at a low and high drive level. The major component of motion was in the x direction. The distortion (elongation) of a given feature was twice the am­plitude of the face-shear motion; thus, the am­plitude of motion was found to be apprOximately

1000± 500 A. utilization of time-resolved SEM would reduce this error. A similar value was found in the case of thickness shear on AT-cut crystals.

1H. Fukuyo, Bull. Tokyo Inst. Technol. Ser. A. 1, 1955.

21. Koga, Y. Tsuzuki, S. N. Witt, Jr. , V. K. Woodcox, R.B. Belser, and W. H. Hicklin, Signal Corps Contract No. DA-36-039 SC-78905, 1960 (unpublished).

3G. Sauerbrey, Z. Physik 178, 457 (1964). 4C.E. Wagner and R.A. Young, J. Appl. Cryst.~,

39 (1969). :>r.E. Everhart, O.C. Wells, and W.C. Oatley, J.

Electron Control 7, 97 (1959). 6R.S.G. Goosd;ves, G.V. Spivak, A.E. Lukianov,

M. V. Bicov, and G. V. Saparin, Proceedings of the Fourth National Conference on Electron Microprobe Analysis, Pasadena, Calif., 1969, p. 58 (unpublished).

7N.C. McDonald, G. Y. Robinson, and R.M. White, J. Appl. Phys. 40, 4516 (1969).

8T. E. Everhart, Symposium on the Scanning Electron Microscope, IITRI, Chicago, 1968, p. 1 (unpublished).

ANNEALING OF ELECTRON-IRRADIATED CdS

Michiharu Kitagawa and Toshio Yoshida The Radiation Center of Osaka Prefecture, Shinke-cho, Sakai, Osaka, Japan

(Received 26 October 1970)

Isochronal annealing of radiation-induced changes in the conductivity, the carrier concen­tration, and the Hall mobility of CdS crystals has been studied over the temperature range 90-410oK. High-conductivity crystals were irradiated by 10-MeV electrons near liquid­nitrogen temperature. The conductivity a, the carrier concentration n, and the Hall mobility p. are decreased by irradiation. Three distinct annealing stages IN' lIN, and lIIN were found. The reverse annealing process slightly occurs in stage IN, 100-130 oK. Appreciable normal recovery occurs in stages lIN' 130-230 OK and IIIN, 330-410 OK. After the annealing at 410 OK the percentages of total recovery of a, n, and p. amount to 74, 78, and 94% of the changes, respectively. The recovery in stage lIN and stage IIIN suggests that two kinds of induced ac­ceptors which also act as charged scattering centers begin to migrate in the temperature ranges; one moves in stage lIN' and the other in stage IlIN.

It has been found that the recovery of CdS irra­diated by fast electrons near liquid-nitrogen tem­perature consists of three distinct annealing stages above 77 OK.

Ultra high pure grade crystals of CdS obtained from Eagle Picher Co. were used in this experi­ment. Two specimens 104-CG and 106-CG, 0.98 mm thick, were formed into "bridges" style and etched chemically in hydrochloric acid at room temperature. Each arm was painted with In-Ga alloy as metal electrodes. Prior to irradiation the sample 106-CG was heated for 40 min at 105°C in nitrogen atmosphere. Before irradiation the car­rier concentration of both samples was 1.46 x 1015

cm- 3 at 77 OK. The samples were irradiated with 10-MeV electrons from a linear accelerator in a liquid-nitrogen cryostat. After every minute of the irradiation the electron beam was turned off to

prevent the samples from increaSing in tempera­ture. The temperature of the samples during ir­radiation was maint!:.ined below 100 ° K.

Figure 1 illustrates the isochronal annealing curves of conductivity cr. Those of the carrier con­centration n and the Hall mobility IJ. for 106-CG are shown in Fig. 2. Total irradiation doses for 104-CG and 106-CG were about 6 x 1015 and 1. 1 Xl 016

electrons/ cm2• The removal rates of carriers

were - O. 2 and - 0.14 (carriers/cm3) per (elec­

trons/ cm2) for 104-CG and 106-CG, respectively.

The conductivity and the Hall coefficient were measured at 77 ° K in the dark after 20 min at each annealing temperature. The carrier sign of the samples remained negative in the experiment. Each point in the figures represents the average of five values obtained under different current den­sities. The experimental error of each point is al-

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