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Zone Melting and Recrystallization

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CRYSTALLINE FILMS ON AMORPHOUS SUBSTRATES BY ZONE MELTING ANDSURFACE-ENERGY-DRIVEN GRAIN GROWTH IN CONJUNCTION WITH PATFERNING

Smt~), Gi(b) Th(pon(cHenry I. Smith(a), M. W. Geis), C. V. Thompson(c), and, C. K. Chen(b)(a) Dept. of Electrical Engineering & Computer Science, Massachusetts

Institute of Technology, Cambridge, MA 02139(b) Lincoln Laboratory, Massachusetts Institute of Technology, Lexington,

MA 02173(c) Department of Materials Science & Engineering, Massachusetts Institute

of Technology, Cambridge, MA 02139

ABSTRACT

Two approaches to preparing oriented crystalline films on amorphous

substrates are reviewed briefly: zone-melting recrystallization (ZMR) and

surface-energy-driven grain growth (SEDGG). In both approaches patterning

can be employed either to establish orientation or to control the location of

defects. ZMR has been highly successful for the growth of Si films on

oxidized Si substrates, but its applicability is limited by the high

temperatures required. SEDGG has been investigated as a potentially

universal, low temperature approach. It has been demonstrated in Si, Ge, and

Au. Surface gratings favor the growth of grains with a specific in-plane

orientation. In order for SEDGG to be of broad practical value, the mobility

of semiconductor grain boundaries must be increased substantially. Mobility

enhancement has been achieved via doping and ion bombardment.

This paper is a brief review of two approaches to preparing oriented

crystalline films on amorphous substrates: zone-melting recrystallization

(ZMR) and surface-energy-driven grain growth (SEDGG). Both have been

investigated for a number of years at M.I.T., the former primarily at M.I.T.

Lincoln Laboratory, the latter on the M.I.T. campus.

ZONE-MELTING RECRYSTALLIZATION (ZMR)

The ZMR process has been applied to a number of film/substrate

combinations [1-5]. The greatest success has been achieved with Si films on

Mat. Res- Soc. Symp. Proc. Vol. 53. 1986 Materials Research Society






substrates of oxidized Si. In unseeded Si ZMR the films obtained have t100]

crystallographic texture and (100> directions within about + 250 of the zone

melting direction [4,5]. (The reason that {1OO] texture dominates is not

entirely clear, but the phenomenon is quite fortuitous since this is the

preferred orientation for MOS field-effect transistors.) Although one can

seed ZMR through openings in the SiO2 to the underlying Si substrate, such

seeding is not necessary in order to achieve single-crystal films. A single

crystal film can be readily obtained without seeding by passing the

solidification front through a narrow constriction [6]. However, azimuthal

orientation is not specifically controlled. Patterning has also been used to

restrict the range of azimuthal angles [7].

For the past few years, the major issues in ZMR studies have been the

origin of defects such as subboundaries, warpage, protrusions and wafer slip

[8-14]. Improvements in the zone melting apparatus have greatly reduced

problems of warpage, slip and protrusions [12,141. However, controversy

still persists relative to the causes of subboundaries and whether it is

possible to eliminate or entrain them.

SUBBOUNDARIES

The solidification front in Si ZMR is faceted, with the slowest growing

1111) facets facing the melt. For the temperature gradients typically used

in Si ZMR, faceted growth is normal; one does not need to invoke cellular

growth due to constitutional supercooling to explain it [11,15]. Forward

growth of the [1111 facets occurs through the nucleation of new ledges

followed by a rapid lateral propagation of the ledges across the facets.

Subboundaries as well as other types of defect trails arise at the interior

corners of the faceted front [3,5,111. Several types of subboundaries and

defect trails are commonly observed, including: (a) branched subboundaries,

(b) unbranched subboundaries, (c) lines of distinct screw dislocations, (d)

linear arrays of dislocation clusters, and (e) dislocation bands [14]. Types

(a), (b), (d) and (e) can be seen in Fig. 1. Type (a) have been reported by






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all workers in the field. Pfeiffer et al. [15] have shown that the charac-

teristic branching pattern that the interior corners trace out (Fig. la)

can be simulated using a computer by assuming that the probability of nu-

cleating new ledges on the (1111 facets facing the melt is linearly propor-

tional to undercooling. (This probability should, of course, be exponen-

tially dependent on undercooling.) The simulation predicted that the

spacing of the interior-corner traces is proportional to the square root of

velocity, in agreement with experiment [5]. This effectively ruled out

constitutional supercooling as an explanation for the faceted interface.

Straight, unbranched subboundaries (Fig. 1 (b) ) were predicted if nucle-

ation occurred primarily at the interior corners.

100 pm(a) (b)

Figure 1. Optical micrographs showing several types of subboundaries anddefect trails. (a) Subboundaries with frequent branchings, type a; (b)The two dark lines are type b subboundaries; the one on the right isbranched, but this is rather uncommon for this type. The three adjacentrows of dots are dislocation clusters (type d). The defect trail betweenthe two type b subboundaries is probably a type e.






6

The question of why there are defect trails and subboundaries emanating

from the interior corners was not resolved by the computer simulation. The

absence of clear evidence of impurity buildup at interior corner traces

[14], together with certain characteristics of subboundaries and defect

trails argue that they are caused by stress due to temperature gradients and

differential thermal expansion [14]. Thermal stress during ZMR is known to

cause slip in the substrate Si wafer, and on this basis alone one should

expect stress to cause dislocations at the interior corners of the solidi-

fying thin Si film.

If certain care is taken in Si ZMR, regions of a film can be found in

which the traces of the interior corners are virtually free of dislocations,

and many of the dislocations that remain scattered throughout the film can

be removed by annealing [14]. However, because of the heterogeneous nature

of the sample, it is questionable whether it will ever be practical to

eliminate dislocations altogether.

An alternative to eliminating defect trails in ZMR Si films is to

entrain them. A variety of methods have been employed [11,14,16]. Some

involve modulating the temperature contour at the trailing edge of the

molten zone. Others, such as the valley entrainment technique shown in

Fig. 2, trap dislocations in the thinner sections of the film. In our

experience, none of the entrainment methods is 100% reliable. In general,

the natural period of the subboundaries must be within about 50% of the

period of the entrainment stripes.

The necessity of producing a molten zone implies that the ZMR process

temperatures must be high, and this, in turn, imposes constraints on sub-

strate materials. For example, three-dimensional configurations become

extremely difficult to fabricate, and in some cases impossible. Because of

the instability of a molten zone against beading, an encapsulation layer

must be used, and the sum of the wetting angles for cap and substrate must

be less than 1800 [17]. In the case of Si, it is fortuitous that a small






Figure 2(a) Cross sectionof the valley entrainmentstructure drawn to scalefor a 2-jim-thick Si film.

VALLEY ENTRAINMENT

VALLEY ENTRAINMENT

Si3N4

U1pro

INPUT END

Figure 2(b) Illustrationof the results with valleyentrainment. The patternextends about 1 mm in thedirection of zone motion.

OUTPUT END

-- •- 50 Aim

residual of nitrogen at the interface between molten Si and the Si0 2 cap

reduces the wetting angle from slightly above 900 to slightly below 900

[17].

For materials other than Si it has proven difficult to find substrates

and encapsulation layers that achieve wetting without introducing other

undesired properties. Compound materials such as InSb [18] and GaAs

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[19] have been prepared by ZMR. However, because of the vapor pressure

differences between the constituents of a compound in the molten state

control of stoichiometry is likely to be a persistent problem.

In summary, the mechanisms of growth of Si crystal films by ZMR are now

fairly well understood. There is strong evidence that subboundaries and

defect trails are due to thermally induced in-plane stress and not impurity

effects. For preparing crystalline films on amorphous substrates, the major

shortcomings of ZMR are: (1) the temperature is too high, (2) molten films

are intrinsically unstable and will break up unless an appropriate encapsu-

lation layer is used, (3) the ZMR process may not be universally applicable,

and (4) for compound materials stoichiometry is likely to be a problem.

SURFACE-ENERGY-DRIVEN GRAIN GROWTH (SEDGG)

Another approach to forming crystalline films on amorphous substrates

is also being investigated which does not involve melting, and has the

potential of working at low temperature and being universally applicable.

The approach is called surface-energy-driven grain growth (SEDGG). As

illustrated in Fig. 3, if a deposited film is sufficiently thin, grains

will extend entirely through it, and will differ in surface energy. Hence,

there is a driving force favoring the growth of those grains that have

Secondary Grain Growth

h dsFigure 3 Schematic depiction ofsurface-energy-driven grain growth(SEDGG). A grain with minimumsurface energy, Ymin., is shown

A)( Ymin- 7growing into a matrix of normalThe driving force due to surface energy anisotropy grains with average surface energy

Y-2AA)_- 2A)(

Ah -h

To get large grains with uniform texturedecrease the film thickness






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minimum surface energy [20,211. This contribution to the total driving

force for grain growth is inversely proportional to film thickness. For

this reason our experiments are on ultrathin films, of the order of a few

tens of nanometers thick. If grain-boundary mobility is sufficiently high,

grains with surface-energy-minimizing orientations should consume other

"normal" grains, leading to a film composed of large secondary grains with

uniform or restricted crystallographic texture (i.e., a specific set of

crystallographic planes parallel to the substrate surface). Since all

crystalline materials have anisotropic surface energy, it is likely that

SEDGG is universal. Given the proper circumstances, it may be possible to

get oriented crystalline films of any material on an amorphous substrate.

To achieve an in-plane or azimuthal orientation in the secondary grains

one must introduce some kind of in-plane anisotropy. One such method,

illustrated in Fig. 4, is to etch a fine surface-relief grating in the

substrate surface. Those secondary grains that have an in-plane orientation

relative to the grating such that their interfacial energy is minimized will

grow to consume grains having other orientations. This method works well

SURFACE-ENERGY DRIVEN Figure 4(a) Schematic cross sec-tion of a film undergoing SEDGG.IO-IOOnm The grain with minimum interfacial/, - -• energy, by virtue of its orienta-1tion, grows by consuming grains

with other orientations.

Figure 4(b) SEDGG in conjunctionwith surface patterning (solid-

IO-IOOnm state graphoepitaxy). A grain ofT Hminimum interfacial energy is onethat is oriented relative to thesurface pattern as well as thesubstrate normal.






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for liquid crystals leading to single-domain films [22]. In the case of

solid films, the SEDGG process has been demonstrated for Si [20,23], Ge

[24,25], and Au, [26,27], and in-plane orientation induced by surface

gratings has been demonstrated for Ge [25] and Au [26].

In the case of Au, secondary grain growth occurs during film deposition

as soon as the film becomes continuous, and continues at room temperature

after film deposition is stopped [27]. The secondary grains have [1111

texture, whereas normal grains have random orientations. The texture seen

in many vacuum-deposited metal films may be the result of SEDGG in the early

stages of film formation. If so, SEDGG is far more common, and hence far

more important, than previously realized.

The problems with the SEDGG approach to forming single-crystal films on

amorphous substrates are: (1) when thermal annealing is used to induce

grain boundary motion the temperature is too high, at least for the case of

covalently bonded semiconductors; (2) a film can minimize its surface energy

by the alternative route of agglomeration (i.e., beading) rather than sec-

ondary grain growth if surface diffusivity is sufficiently high; (3) align-

ment induced by surface-relief gratings has an angular spread of many de-

grees.

To overcome these problems we are pursuing means of enhancing the

grain-boundary mobility at low temperature. Efforts in this direction

include experiments on the effects of dopants, ion bombardment and intense

illumination. Except for the latter, these experiments are reported else-

where in this conference [28,291. The experiments on the effect of dopants

have been done in Si thin films with P, As and B as dopants [23,28]. It was

found that doping with P and As leads to greatly enhanced rates of secondary

grain growth and apparent increase in grain boundary mobility. Compensation

of P doping through codoping with B can lead to reduction or elimination of

enhanced grain boundary mobility, suggesting that the mobility is a function

of the electron concentration.






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