seed window defects in silicon on insulator material

014 Applied Surfa,:e Science 36 (1989) 614-622 North-Holland, Amsterdam

S E E D W~NDOW DEFECTS IN S ILICON ON I N S U L A T O R MATERIAL

D.A. WILLIAMS, R.A. M c M A I t O N and H. A H M E D

Microelectronics Research Laboratory, Department of Physics, Cambridge University, Science Park, Milton Road, Cambridge, CB# 4FW, UK

Received 2 lune ]o~,8; accepted for publication 17 June 1988

Sec~cled zone melt recrystallization has been conducted using a dual electron beam system to form layers of silicon on insulator. The quality of the resulting single crystal films has been analysed by a variety of microscopic techniques, including cross sectional scanning and transmission electron microscopy. Defects were found in the seed windows which are attributed to the presence of an oxide layer at the original boundary between the single crystal seed and the deposited layer of polycrystaUine silicon. The postulate that these defects, principally dislocation clusters, are due to thermally induced stresses during reerystallization, has been ruled out by the use of material where no oxide layer is present and which is defect free in the set, d windows.

l. In~o~uefion

Silicon on insulator structures, in which layers of silicon are formed on an electrically insulating substrate, have many applications in microelectronics [1], and have been the object of many experimental investigations. A large number of techniques have been developed for the formation of such films [2], and that used for this study was the seeded zone melt recrystallization of a layer ofpolycrystall ine silicon by a dual electron beam technique [3].

The investigations reported here concentrated on the seed window regions of the films prepared by dual electron beam recrystallization. As the orientation of the film and its material quality require good epitaxy from the seed, this region is of particular interest. It was found that dislocation tangles and other defects were present in the seed windows in some specimens, and these are attributed to residual oxides at the origir, al interface between the deposited polycrystalline silicon and the single crystal seed. These may be remnants of the isolating oxide, not fully removed, or may have grown between the definition of the seed windows and deposition of the polycrystalline silicon.

The usual procedure for making electren_;c devices in silicon on insulator is to form a film of single crystal silicon on top of an insulator, and then remove unwanted regions of that film by lithography and etching to leave isolated islands or mesas of single crystal. An individual device is fabricated in each island, and then islands are linked by metallic connections to give a finished

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D.A. Williams et aL / Seed window defects in silicon on insulator mater~al 615

ci~-cuit [4]. The fact that each device is m.herently electrically isolated from its neighbours means that there is less capacitive linking or resistive leakage between devices, which allows for higher packing densities and faster circuit operation than in conventional integrated circuitry. Ti~e technique also allows the mixing of device technologies in one chip, and in logic circuitry removes CMOS latch-up (the turning on of a parasitic n - p - n - p structure in conventional CMOS circuits, which locks a logic gate in one state). The particular application of interest to this study is the formation of three-dimensional integrated circuits, which is achieved by stacking layers of devices with vertical and horizontal interconnections [5]. For this application, it is particularly important that the recrystallization of an upper layer does not adversely affect devices which will have been fabricated in underlying regions. Thus it is very important to study the thermal profile and mechanical stresses during the recrystallization process. Tiffs may be done with single layers of silicon on insulator as a precursor to more exhaustive studies on fully stacked three-dimensional structures, and this investigation has used single layers to study se~d window phenomena.

The structare used in these experiments had a single crystal silicon wafer of (001j o~k~r, tation as a substrate and crystal seed. A layer of silicon dioxide 1.0 ,urn thick was deposited on this, by chemic~ vapeur deposition at 400 ° C, and seedin~ windows cut through to the silicon.in an array of parallel lines 2-5/~m wide, spaced by 20-100 ptm. A layer of polycrystaUine silicon 0.5-I .0 /~m thick wa,~ deposited over this, and the structure capped with a composite of 0.3 /zm undoped and 0.7/~m phosphorus doped silicon dioxide.

Dual electron beam recrystallization is performed using one ~ ectron beam rapidly raster scanned over the back surface of the wafer to give uniform heating in the range 700-1000 ° C. A second beam, of approximately Gaussian profile with ?WHM -= 100/~m, is scanned wi:h a triangle wave source at 100 kHz to synthesize a line heat source. This is scanned over the front surface of the wafer, ef~ecting recrystallization by melting the polycrystalline silicon, which subseavently refreezes seeded from the substrate to give a film of uniform single crystal silicon.

Analysis of the recrystallized silicon is perforrhed using several microscopic techniques. Tho~:e of most use for this investigation were cross-sectional scanning and trancmission electron microscopy. For scanning microscopy, the sample is cleaved then etched in buffered hydrofluoric acid to delineate the oxide layers and in Secco etch to show material defects. It is then sputter coated with gold/palladium prior to observation in a Hitachi S-800 scanning electron microscope. For transmission microscopy, two samples are glued together face to face, then mounted w,~th epoxy resin in a steel and brass support structure with a total outer diameter of 3 ram. Slices are cut from this with a low deformation saw, mechanically thinned, dimpled and ion milled before observation in a Philips EM-300 transmission electron microscope.

616 D.A. Williams et al. / Seed window defects in silicon on insuhitor material

2. Results

After recrystallization, the silicon film was found to be single crystal, with only one subgrain boundary running down the middle of the film above each isolating oxide bar, equidistant from adjacent seed windows [6]. This has been shown to be caused by the differences in the coefficients of thermal expansion between the isolating silicon dioxide and the silicon film [7], resulting in a twisting of the film. This strain is accommodated in the few microns of the film near the seed windows, and most of the film is strain free, but the material regrown from each seed window is misoriented from the other by 0.5 °-1° . This misorientation is variable in magnitude between nearby st~pes, and is taken up by a subgrain bouncb_.~' c o m p o s e d principally of edge dislocations.

It has also been observed that there were often dislocation tangles, or isolated dislocations in the seed window region. These always occurred in material which had melted and regrown, even when in the bulk silicon substrate. (Melting 1 or 2/~m into the bulk silicon in the seed window region is necessary for seeding.) It had been postulated that these were due to thermal strain, but the lack of repeatability and the character of the tangles when observed tended to suggest otherwise. The thermal cycle of the material is very well characterised and repeatable, giving rise for example to the central subgrain boundaries which are uniform in character and repeatably con- strained to the central few microns of the regrown film by the thermal flow.

The quality of the recrystallized silicon in the seed windows is not usually important for applications requiring one level of silicon on insulator. However, the defect structures observed there give information on regrowth mechanisms, and it is also important to ascertain the effect of rccrystallization of an upper layer on lower material which w~uld contain devices in a three-dimensional circuit application. In addition, defects in the seed window region may propagate into the film which is to be used for devices, and may be indicators of factors affecting recrystallLzation.

Fig. 1 shows a transmission electron micrograph of a dislocation "staircase" structure at the end of an oxide bar, and fig. 2 a scanning electron micrograph of a similar region. The small etch pits in fig. 2 are where each dislocation intersects the surface. This also demonstrates the use of high resolution scanning electron microscopy, as the features observable are the same as in transmission microscopy, with the exception of twinning, but specimen pre- paration is much faster.

The dislocations seen are not usually in the regular structure seen in fig. 1. Fig. 3 shows a cross-sectional transmission electron micrograph of a specimen where only a few isolated dislocations are to be seen, and fig. 4 shows a dislocation tangle. The features seen usually occur regularly within one wafer, but will vary from wafer to wafer, even for idenffcal processing parameters.

D.A. Williams et al. / Seed window defects in silicon on insulator material 617

Fig. 1. Cross-sectional TEM showing a ladder type array of seed window dislocations at the end of an oxide bar.

Thi~ suggests that the material is different in some respects from wafer to wafer. Dislocation tangles are seen in the seed window region of material where the oxide was grown by localized oxidation, rather than deposited, showing that the same effect may occur for various types of isolating oxide. The character of the defects is the same for seed window alignment on (100) and (110) directioas, in contrast to the defects observed in the film.

Initially, these defects were attributed to thermal stress during recrystallization, as the silicon and silicon dioxide have different coefficients of thermal expansion. This appears to be the origin of the central subgraJn boundary, which on average takes up less misofientation (i.e. is made up of fewer dislocations), when recrystallization is performed at a higher background temperature. In this case the thermal gradient through the layers is lower, and less stress is expected. However, the seed window dislocations are less well

618 D.A. Williams et al. / Seed window defects in silicon on insulator material

Fig. 2. Cross.sectional SEM showing a similar ladder type array to fig. 1.

Fig. 3. Cross-sectional TEM showing isolated dislocations.


Fig. 4. Cross-sectional TEM of a dislocation tangle in a seed window region.

behaved, and seem to have no such dependence on background temperature. In addition, the defects are always seen in regrown material, even when full melting of the seed window region does not take place. This suggests that ~hey are due to something interfering with ~,e smooth growth of silicon. The dislocation tangles observed are reminiscel~t of oxide related defects, as are other defects observed in the regrown film such as platele~s which are seen when seed window defects are present. The defects in the regrown film itself will be discussed elsewhere.

The presence of unwanted silicon dioxide at the interface between the bulk silicon seed and the deposited silicon in the seed window has been known to cause major problems in seeded recl2cstallization [8]. Fig. 5 shows an extreme case where the seed window has not been fuUy opened, arid so no seeding is possible. This is not always obvious when making preliminary inspections of the silicon surface with optical microscopy alter defect etching. The seed windows act as heat sinks, constraining the regrowth front to the same geometry as it would have if the seed window were open [6]. This leads to the same character of regrown material as if it were seeded with (001) vertical texture but of random lateral orientation. Thus cross-sectional microscopy is necessary to characteri:'e fully the m~terial. Fig. 6 shows the case of a very thin layer of oxide wiuch is not observable even in high resolution scanning electron microscopy. It has prevented seeding, so that even though the silicon above the layer has all naelt~, it is ~o~3~sta'dine. A layer of this thickness will break up if the beam power is high enough, b~;t then many defects are

620 D.A. Williams et at,. / Seed window defects in silicon on insulator material

Fig. 5. Cross-sectional SEM showing a relativcly ayer of oxiae completely preventing see~

s~:en both in the seed window and in the silicon film. It is believexi, that thin iayers of silicon dioxide such as this, but not necessarily cominuous, are responsible for the defects seen in silicon on insulator seed windows. The defects would be expected to initiate at sites such as small precipitates which interrupt the smooth progression of the regrowth front (all the defects seen in zone melt recrystallized silicon on insulator are growth related, and so depen- dent upon the morphology of this front). Although no such precipitates have been observed, the probability of finding one in a particular transmission electron microscope sample of thickness - 1 0 0 0 A is very small, and not

i II

Fig. 6. Cross-sectional SEM showing a very much thinner layer of oxide still preventing seeding.


Fig. 7. Cross-sectional TEM of material where the polycD'shalline silicon was deposited immediately after in situ cleaning of the seed windows. 'I'here are no seed window disloea~;~oas.

enough samples have been studied for this lack of observation to be statisti- cally significant. It is r, ol c'.ear whether the etching technique would dis- criminate between a small precipitate of oxide and the intersection of a dislocation with the sample surface, and so it is possible that such r, reclpitates have been observed i,,~ ~eanning electron microscopic analysis, but incorrectly identified.

When material is used in which the polycrystall/ne silicon was deposited immediately after an in-situ etch with HCI gas [8], no seed w/ .dow dislocations are seen. This is true for the range of background temperatures studied (700-1000 o C), and for all line power densities which give seeded recrystaiiiza- tion but which do not melt the silicon under the isolating oxide. Fig. 7 shows such a sample. This is conclusive evidence that the cleanliness of the polycrystal-seed silicon interface is of great importance in seeded zone melt recrystallization, and is the sole cause of seed window defects in electron beam recrystallized silicon on insulator.

When the power density in the line electron beam is increased to a level where several microns of melting occurs within the bulk silicon under the isolating oxide, many dislocations are observed ir~ both the regrown and bulk material. These are attributed to thermal strain, and may gi~e rise to wafer slip and warpage. However, this condition is not accessed in normal ~'ecrystalliza- tion.

622 D.A. Williar~ et al. / Seed window defects in silicon on insutator material

3. Conduslnns

Dislocation structures are often found m the seed windows of zone melt recrystallized silicon on insulator material. They are caused by impurities, principally residual oxides, at the original polycrystal-seed silicon interface and not by any strain mechanism. They may be removed entirely by attention to the cleanliness of the interface. A further discussion of the role of oxides and other impurities in electron beam zone melt recrystallization will be published elsewhere.

Acknowledgements

We acknowledge the support of the ESPRIT project 245 for part of this work. D.A.W. ackno~vledges the support of a postdoctoral research fellowship from the U K Science and Engineering Research Council.

References

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seed window defects in silicon on insulator material

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