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HRTEM INSITU OBSERVATION OF GRAIN BOUNDARY MIGRATION OF SILICON Σ
BOUNDARY AND ITS STRUCTURAL TRANSFORMATION AT 1000K
H. Ichinose, Y. Ishida
To cite this version:
H. Ichinose, Y. Ishida. HRTEM INSITU OBSERVATION OF GRAIN BOUNDARY MIGRATION
OF SILICON
ΣBOUNDARY AND ITS STRUCTURAL TRANSFORMATION AT 1000K. Journal
de Physique Colloques, 1990, 51 (C1), pp.C1-185-C1-190. �10.1051/jphyscol:1990128�. �jpa-00230286�
:OLLOQUE DE PHYSIQUE
:olloque Cl, supplbment au n o l , Tome 51, janvier 1990
HRTEM INSITU OBSERVATION OF GRAIN BOUNDARY MIGRATION OF SILICON 1 3 BOUNDARY AND ITS STRUCTURAL TRANSFORMATION AT 1000K
H. ICHINOSE and Y. ISHIDA
Institute of Industrial Science, University of Tokyo, 7 - 2 2 - 1 Roppongi Minato-ku, Tokyo, Japan
Abstract-The atomic process of both the grain boundary migration and the structure transformation of (111)A/BD and (112)A/BD CSL boundaries in silicon was investigated in-situ by the HRTEM a t 1000K.
A unit distance of the boundary migration was not one plane distance. It coincided with the CSL lat- tice constant; T h e boundary migrated not by successive replacement of atoms of the next plane but by the exchange of atomic site from that of one crystal to the other's in the volume which corresponds to a unit CSL cell next to the boundary. Owing to thc coincidence of the unit migration distance with the period of the CSL neither the strain in the vicinity nor the disturbance of periodicity in the neigh- boring (112)A/BD boundary resulted by the migration.
A non periodic (112)A/BC3 boundary changed during heating into a periodic one a t around %OK.
Both periodic and non periodic structures appeared in turn when the temperature was kept at 1000K.
Although so many experimental data on the grain boundary migration were accumulated in the past 30 years the kinetics of the grain boundary migration is still remained to be not obvious. Present situation is attri- buted to two causes; The methods which have been employed for a long time to measure the migration distance as a function of temperature were rather indirect to investigate the kinetics. No structural feature of individual boundaries was taken account even though it is deeply related to the atomic process of the grain boundary migration as it was redicted by Gleiter(1). By this reason obtained data tend to be dispersed.
Recent computer simulation(27 may provide an information about the atomic process of the grain boundary migration but it needs a n experimental proof.
Recent high resolution electron microscopy enables us to direct observation of atomic process of the grain boundary migration process as well as the structure transformation of the grain boundary. Ichinose and Ishida(3)(4) firstly observed the structural change which was accompanied by the grain boundary migration in gold thin foil by the lattice image in-situ observation. In that case the heat to activate the structural change was provided by the electron beam irradiation in the high resolution electron microscope itself.
In the present study a poly -silicon specimen was heated in the high resolution electron microscope which was equipped with a specially designed high resolution heating stage and the in-situ lattice imaging obser- vation was done to investigate the atomic process of both the grain boundary migration and the s t r u c t ~ ~ r a l transformation. T h e highest temperature was determined to be lOOOK which roughly corresponded to T m / 2 of silicon.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990128
COLLOQUE DE PHYSIQUE
(a) The HRTEM heating stage
A high resolution heating stage system which consists of a top entry heating specimen holder, high stability power supply and a tilting goniometer are specialy designed and produced by JEOL company to put the JEM-200CX HRTEM. A simple electric current heating is employed in order to keep high stability as well as high reliability. The heater consists of a fine double spiral tungsten coil designed for heating up to 1100K.
Temperature is monitored by a thermo-couple which is put nearby the heater. The accuracy of monitoring the temperature is checked by the melting test of tin to be
*
l0degree.In order to keep the high resolution the objective lens of the microscope is also newly designed. The resulted aberration constants (C,=l.Omm and CC=1.2mm) promise the same level of resolution as ordinary high re- solution JEM-200CX at temperatures up to 1100K. Heating current is supplied by regulated direct current power supply through the regulated current controller. Stabilit of the power supply is estimated to be 10-6.
Neither the suspected local disturbance of the magnetic field orthe objective lens due to the heating current nor the mechanical vibration due to mutual (as well as self) induction of the heating coil caused by the resi- dual ripple component of the heating current influence on the resulted resolution.
(b) Specimen preparation,thinning and lattice imaging
Polycrystalline silicon is produced by the evaporation of high purity (99.999%) silicon onto silicon substrate in the vacuum of W 7 P a up to few hundred nanometer in thickness. The <110> axis of many crystals is per- pendicular to the foil surface to produce a pure tilt boundary about <110> axis.
An ion bombardment method is employed for the thinning of the specimen. The specimen is observed with the symmetrical illumination in the direction parallel to the common <110> axis. Diffraction waves up to (200) are allowed to pass the objective aperture.
(c) Image recording and processing
The image recording was done by video tape. In order to detect the structural changes pictures are producecl from the video tape. The interval of successive pictures was lOsec arid an exposure time is 1/12sec which is far shorter than ordinary photographing. The detection of the structural change is done by the overlapping of two successive pictures which are printed onto transparent sheet.If there is a change in atomic configuration it is recognized by local mismatching between two pictures otherwise the two pictures overlapp perfectly.
3-RESULTS AND DISCUSSION
An arrangement of white spots of the lattice image represented the atomic arrangement of a silicon crystal well even though each spot did not correspond to individual atomic column position; A white spot in the micrograph which was taken at o timum defocus condition corresponded to a tunnel which was produced by an atomic hexagonal ring. ~ h e r e g r e , an arrangement of white spots was same as that of fcc structure. The reason why the spot does not represent each atomic position is that the resolution of a current HRTEM is not enough to resolve O.14nm which is the closest spacing of the two atomic column positions in silicon viewed along <110> direction.
3-1 Migration of (111)A/B5 CSL boundary (a) Migration process
Since both the grain boundary migration and the structural transformation advanced slowly and the amount of the migration was of atomic scale a t 1000K, it was possible to follow the atomic process of the structural change by comparing a series of pictures by the method mentioned in the previous chapter. The observed region of the specimen is overviewed in figure 1. Two crystals were connected by the C3 CSL relation (two crystals were rotated by 70.5degree about <110> axis). The interface of the crystals consisted of (lll)A/BY3 and (112)A/BC3 CSL sections connected in turn. The detailed rain boundary migration process was ob- served at the longest ( I l l ) A / B 5 segment (CT1). A short ( l l $ A / ~ 5 segment (CT2) also migrated.
The boundary migrated not by successive replacement of atoms of the next plane but by repetition of a pro- cess which consisted of three steps; At first, In the picture, all white spots on the boundary and a half of the next row were broadened and made brighter than that of the other region in figure 2(a). The broadened spot size may be explained by the change of Debye Wallerfactorand the corresponding increase in the amplitude of the vibration. Secondly, the white contrast of the two rows next to the boundary weakened and became diffuse as in figure 2(b). Thirdly, after short duration of time the diffused contrast cleared again and at the same time the grain boundary appeared to have moved to the third row as in figure 2(c).
(b) Migration mechanisms
The characteristic feature of the presently observed migration is that the migration unit is not a single crystal plane distance but rather that of three atomic planes which corresponds to a unit cell distance of 5 CSL.
If the grain boundary migrated by one atomic plane distance it needs a rigid translation of a component crystal by the amount of a/6<112> in the direction parallel to the interface as shown in figure 3. The larger
the translation the longer the migration distance. But this is not the case because nothing of rigid translation was detected even by the overlapping of successive pictures.
In order to migrate the boundary in the presently observed manner rearrangement of atoms from the lattice site of the one crystal to the other's is needed in the region which is swept by the boundary (figure 4(a)(b)).
The observed diffusing of the contrast in the two atomlc rows next to the boundary (figure 2a) may have in- dicted the following process. (a) Displacements of atoms from ordinary positions. Decreased flatness of crystal planes due to the atomic displacement shall cause the weakening of diffractions which contribute to the lattice images. While (b) The migration of atoms in the region. If all atoms in the region migrate there is no stable crystal plane to reflect the electron wave any more. Instantaneous amorphization is also included in this category. Although required the atomic rearrangement is ex ected in either case the former is more favorable. Because, as it is seen from the CSL plot of E3 in figure 4($, the site exchange due to a/( 112, type displacement of each atoms is most reasonable. Detailed atomic process of the site exchange whether the atomic displacement is initiated a t one corner and propagated in the region or all atoms displaced simul- taneously is still remained unknown.
The advantages of the present migration process is firstly that it doesn't need any rigid translation of a com- ponent crystal and secondary that the riodicity of the adjacent (112)A/BE3 boundary was preserved during the migration of the ( l l l ) ~ / ~ I ! ? b oundary. Therefore, no energy elevation due to breaking of
periodicity occur.
3 -2 Transformation of (112)A/BE3 boundary
In the present specimen the (112)A/BE3 boundary some times appearsnon periodic at room temperature as shown in figure 5(a). One reason why it remained as disordered is that the specimen was not fully annealed after the production. The structure change was observed in this type of boundary which is shown as CT1 in the figure 1.
In the course of the temperature elevation the boundary changed its structure into a periodically ordered one as shown in figure 5(b) at the temperature at about 950K. The change was abrupt. It occured at least within
1/12 second which is the exposure time of the picture. The periodic structure changed into non periodic one again at about 1000K. After this change the temperature was kept at lOOOK and both the periodic and non periodic structure appeared in turn probably because the 1000K is just at the temperature where the two structure are of comparable free energy.
Figure 1. The observed region. The corrugated interface consists of segment (111)A/BD and (112)A/BC3 CSL boundary connected in turn. Grain boundary migration was observed at CT1 and CT2. The transfor- mation in the interface structure was observed in lT1.
Cl-188 COLLOQUE DE PHYSIQUE
Figure 2(a). The first step of the grain boundary migration of the (111)A/BC3 boundary. All of white spots on the boundary and a half of the next row increased the size and brightness. (b) The second step of the migration. Two rows next to the boundary were decreased in contrast and brightness, lattice image was diffuse. (c) The third step of the migration. When the diffuse contrast became clear again the boundary has appeared to have migrated. Original boundary position is shown by an arrow.
A A A A
Figure 3. (a) Atomic coincidence of W relation of diamond structure (crystal A and crystal B) viewed in the
<110> direction. (b) By the translation of one crystal by a/6<112> in the direction parallel with the (111) plane a well matching row ((111) plane, to be grain boundary) moves to the next plane. (c) Schematic illustration of a (111)A/BC3 boundary structure. (d) By the a/6<112> rigid translation of right hand side crystal the grain boundary is migrated by one (111) plane distance.
Figure 4. (a) Geometrical structure of (11l)A/BD and (112)A/BD boundary. Unit cell of C3 is shown by a dotted line. (b) Due to rearrangement of atoms; which are indicated by arrows, the boundary migrated by the distance of the CSL unit cell. Periodicity of the (112)A/BD boundary is kept unbroken.
Figur 4. (c) CSL plot (atomic coincidence) of the diamond structure. Three possible atomic displacements to exchange the atomic site from that of one crystal to the other's (shown by arows) are of the type of
a/6<112>.
Cl-190 COLLOQUE DE PHYSIQUE
Figure 5(a). (112)A/BD boundary, not yet annealed. (b) At 950K, periodic ordered structure is realized. (c) At lOOOK the structure changed into a non periodic one again. After this both atomic arrangements appeared in turn a t 1000K.
REFERENCES
1. Gleiter H., Z.Metallkunde 61(1970)282 2. Wulf et al., (to be published in Phys.Rev)
3. H.Ichinose and Y.Ishida, Proc.Int.Cong.In-situ experiment of HVEM-Osaka 1(1985)333 4. H.Ichinose and Y.Ishida, Phil. Mag. (in print)
