study of the interface microstructures of cvd diamond films by tem

Study of the Interface Microstructures of CVD Diamond Films by TEM

Alexander G. Fitzgerald1;�, Yongchang Fan1, Phillip John2, Clare E. Troupe2, John I. B. Wilson3,

Anthony O. Tooke1, and Brian E. Storey1

1 Department of Applied Physics and Electronic & Mechanical Engineering, University of Dundee, Dundee DD1 4HN, UK2 Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, UK3 Department of Physics, Heriot-Watt University, Edinburgh EH14 4AS, UK

Abstract. The characteristics of the interface micro-

structures between a CVD diamond ®lm and the silicon

substrate have been studied by transmission electron

microscopy and electron energy loss spectroscopy. The

investigations are performed on plan-view TEM speci-

mens which were intentionally thinned only from the

®lm surface side allowing the overall microstructural

features of the interface to be studied. A prominent

interfacial layer with amorphous-like features has been

directly observed for CVD diamond ®lms that shows a

highly twinned defective diamond surface morphol-

ogy. Similar interfacial layers have also been observed

on ®lms with a h100i growth texture but having the

{100} crystal faces randomly oriented on the silicon

substrate. These interfacial layers have been unam-

biguously identi®ed as diamond phase carbon by both

electron diffraction and electron energy loss spectros-

copy. For the CVD diamond ®lms that exhibit hetero-

epitaxial growth features, with the {100} crystal faces

aligned crystallographically on the silicon substrate,

such an interfacial layer was not observed. This is

consistent with the expectation that the epitaxial

growth of CVD diamond ®lms requires diamond crys-

tals to directly nucleate and grow on the substrate

surface or on an epitaxial interface layer that has a

small lattice mis®t to both the substrate and the thin

®lm material.

Key words: CVD diamond ®lms; interface microstructure; trans-mission electron microscopy; electron energy loss spectroscopy.

A particularly useful application of the transmission

electron microscope (TEM) in CVD diamond ®lm

studies is in the investigation of the characteristics of

the interface between the diamond ®lm and the

associated substrate. Information on interface micro-

structural properties of the ®lms is usually obtained by

cross-sectional TEM (XTEM) observations. In a few

cases, epitaxial growth of CVD diamond ®lms directly

onto certain kinds of substrates such as single crystal

Pt (111) and cubic boron nitride has been con®rmed

by TEM observations [1 ± 2]. However, in the majority

of XTEM investigations, the diamond crystal grains

were not found to be in direct contact with the

substrate. There is an interfacial layer between the

substrate and the diamond crystal, and the structure of

the interfacial layer is strongly in¯uenced by the

deposition conditions.

In some situations, the interfacial layer that exists

between the diamond ®lm and the silicon substrate

has been identi®ed as epitaxial silicon carbide [3 ± 5].

However, in many cases, the microstructures observed

at the interface were found to be more complicated

and the interfacial layer on which the diamond

crystallite can nucleate and grow cannot be simply

classi®ed as silicon carbide [6]. Diamond ®lms have

also been observed to grow on amorphous silicon

carbide [5], amorphous carbon [4] and on `diamond-

like' carbon [7] interfacial layers.

The microstructural information on the interface

between a silicon substrate and a diamond ®lm has

been obtained exclusively by cross-sectional TEM

observations. This type of observation is not easily

performed in practice because of the dif®culties

Mikrochim. Acta 132, 315±321 (2000)

� To whom correspondence should be addressed

involved in the preparation of cross-sectional TEM

specimens. In many cases, the composition and phase

of the observed layer cannot be identi®ed by electron

diffraction or electron energy loss spectroscopy

because of the small cross-sectional dimensions of

the interface layers. Also, the overall microstructural

properties of the interface layer usually cannot be

obtained by cross-sectional TEM observations.

In this paper, we report on microstructural studies

of the interface by plan-view TEM observations. In

these studies, some plan-view TEM specimens were

intentionally thinned only from the ®lm surface side

allowing the overall microstructural features of the

interface to be directly observed. In these investiga-

tions, special attention has been focused on the

correlation of the ®lm growth textures with the

microstructural properties of the interface.

Experimental

The diamond ®lms investigated in this work were prepared on asilicon (100) substrate by a three stage process i.e. carburisation,biasing nucleation and growth in a UHV compatible depositionsystem which has been constructed for the growth of diamond by2.45 GHz microwave plasma enhanced chemical vapour deposition[8 ± 10]. The prepared CVD diamond ®lms can be visualised to beroughly composed of two distinct growth regions i.e. a centralgrowth region with a diameter of about 4 ± 6 mm that has a deepdark colour and an annular ring region with a band width of 4 to5 mm that has a shiny grey colour. AFM observations show that the®lms deposited in these two regions have a different growthtexture. Also, the surface morphology changes greatly with theposition along radial directions within each growth region.

By fracturing the diamond film and substrate, some pieces offree-standing diamond film could be obtained. For plan-view TEMobservations, specimens from these pieces of free-standingdiamond film were sandwiched in a folding copper electronmicroscope grid and then thinned by ion milling with a 5 keVargon ion beam, at 20� incidence until perforation. The free-standing diamond films for TEM specimen preparation wereintentionally chosen respectively from the central growth regionand the annular ring region to enable correlation of themicrostructural properties of the film with corresponding morpho-logical features. Some of the plan-view TEM specimens werethinned by ion etching only from the thin film surface side and thisenabled the microstructure in regions near to the diamond-siliconinterface to be studied. The TEM investigations were performed ona JEOL 200CX transmission electron microscope which wasoperated at 200 kV.

Results and Discussion

Diamond Films with a Highly Twinned

Defective Surface Morphology

Specimen I was taken from the central growth region

in a CVD diamond ®lm. Atomic force microscopy

(AFM) observations showed that the ®lm deposited in

this central region was mainly composed of randomly

oriented microcrystallites (Fig. 1a), and the ®lm

usually showed a highly twinned defective diamond

surface morphology. The corresponding microstruc-

tural features near to the interface for this ®lm are

shown in Fig. 1b and Fig. 1c. An intermediate layer

can be observed to exist in between the silicon

substrate and the ®ne diamond crystal grains. By

observation of the spherical-like appearance of the

deposits in this region, the interfacial layer appears

more likely to be amorphous than crystalline. From

the high magni®cation micrograph shown in Fig. 1c,

the larger round-shaped lumps with a size from a few

tens to several hundreds of nanometres appear to be

aggregated from much smaller clusters or blisters.

Electron diffraction analysis (see Fig. 1d) indicates

that the ®lm that initially grew at the interface region

was in fact polycrystalline diamond in nature. The

®rst four prominent diffraction rings in the electron

diffraction pattern have been indexed as due to

diffraction respectively from the cubic diamond

{111}, {220}, {311} and {222} lattice planes.

Diamond Films with a Predominant

h100i Growth Texture

Specimen II was taken close to the annular ring region

in a CVD diamond ®lm. The ®lm grown in this region

had a predominant h100i growth texture as illustrated

in Fig. 2a. The majority of the diamond crystal grains

formed in this region exhibited square or rectangular

(100) crystal faces which were approximately parallel

to the substrate surface. The existence of an inter-

mediate layer between the well-developed diamond

crystal grains and the silicon substrate can be seen

more clearly in this sample as shown in Fig. 2b to Fig.

2d. From these TEM micrographs, the observed dark

intermediate layer has an amorphous appearance.

However, subsequent electron diffraction and electron

energy loss spectroscopy (EELS) analysis indicated

that this intermediate layer was also mainly composed

of the polycrystalline diamond-phase carbon material.

From these TEM micrographs it can also be seen

that the specimen is not completely covered with this

intermediate layer. For this sample, whether or not the

interfacial layer can be observed in TEM images

seems to depend on whether or not this layer is

attached to the free-standing ®lm or is left on the

substrate during the diamond ®lm-substrate separation

316 A. G. Fitzgerald et al.

Fig. 1. Surface morphology andinterface microstructures for dia-mond ®lm specimen I, the diamond®lm thickness is about 12mm. (a)AFM topographic image of the ®lmsurface, (b and c) TEM micrographstaken from the interface, (d) elec-tron diffraction pattern taken fromthe area shown in (c)

Fig. 2. Surface morphology andinterface microstructures for dia-mond ®lm specimen II, the diamond®lm thickness is about 12mm. (a)AFM topographic image of the ®lmsurface, (b, c and d) TEM micro-graphs taken from the interfaceregion

Study of the Interface Microstructures of CVD Diamond Films by TEM 317

process. This speculation has been con®rmed by AFM

observations on the surface of the detached ®lm which

was in contact with the substrate and the substrate

surface from which the ®lm had been detached. When

the diamond ®lm was separated from the substrate, in

some areas, the interfacial layer is left on the

substrate, in other areas, the interfacial layer is

attached to the separated free-standing ®lm. For just

this reason, the interfacial layer without a diamond

®lm superimposed on it and the diamond layer

without the interfacial layer attached to it can be

observed and studied simultaneously on single TEM

plan-view specimen.

When an area that was not covered with this

intermediate layer was examined at high magni®ca-

tion, some large grain-like areas were observed as

illustrated in Fig. 3a. All of the large grain-like areas

consisted of several fan-shaped growth sections and

the fan-shaped growth sections in each grain-like area

emerged from a common centre within the grain.

Similar growth features near to the interface have also

been observed in other cross-sectional TEM investi-

gations [1, 2, 6]. Since the defects are generated

because of the growth species sitting at improper

lattice sites during the deposition process, the defect

lines in the fan-shaped growth section actually re¯ect

the trace of crystal growth. These observed features

imply that each diamond grain nucleates from a single

site and the diamond crystal grows in three-dimen-

sions [6]. The polycrystalline diamond nature of these

larger grain-like areas has been con®rmed from the

well-de®ned electron diffraction pattern obtained

from these grains shown in Fig. 3b.

For comparison, a high magni®cation TEM image

and corresponding electron diffraction pattern taken

from the intermediate layer are illustrated respectively

in Fig. 3c and Fig. 3d. The diffraction pattern taken

from the intermediate layer (Fig. 3d) is the same as

the pattern taken from diamond crystal grains

(Fig. 3b). So it can be inferred that the sp3 bonded

diamond phase is the main ingredient in the

intermediate layer.

The diamond nature of the intermediate layer has

been further con®rmed by electron energy loss

spectroscopy. The plasmon loss spectra taken from

the intermediate layer is exactly the same as that taken

Fig. 3. TEM micrographs and elec-tron diffraction patterns taken fromspecimen II, (a and b) from grain-like areas, (c and d) from the inter-facial layer


from diamond crystal grains as shown in Fig. 4. The

characteristic loss peak at 34 eV, which is unique to

diamond and caused by bulk plasmon resonance of

valence electrons [11] can be observed to appear in

the spectra from both the diamond grain and the

interfacial layer. The peaks at around 65 eV and 99 eV

are also characteristic of diamond and are due to the

excitation of second and third plasmons. A similar

loss feature has also been observed on the carbon K-

edge electron energy loss spectra. The intermediate

layer that existed at the diamond-silicon interface can

therefore be unambiguously identi®ed as diamond

phase material.

Diamond Films with an Epitaxial

h100i Growth Features

Specimen III was also taken from the annular ring

region in a CVD diamond ®lm but the difference is

that this ®lm exhibited superior epitaxial growth as

shown in Fig. 5a and Fig. 5b. Most of the square-

shaped diamond crystal grains can be seen to be well

Fig. 4. Electron energy-loss spectra in the low energy loss regionof 0 to 150 eV, (a) taken from a large diamond crystal grain, (b)taken from the interfacial layer

Fig. 5. AFM images taken fromspecimen III, the diamond ®lmthickness is about 25 mm. (a and b)Images taken from the ®lm surface,(c and d) images taken from theinterface side of the ®lm, image scansize: (a) 30mm, (b) 10mm, (c) 40 mm,(d) 6.5 mm


aligned with each other with their crystal edges

running in two predominant directions. These features

indicate that the diamond crystal grains, in this case,

grow epitaxially on the silicon substrate. The interface

side of the ®lm i.e. the surface that was in contact with

the silicon substrate before being detached was also

observed by AFM and the representative AFM images

are illustrated in Fig. 5c and Fig. 5d. From these AFM

images it can be seen very clearly that circle-like

marks appear at the centre on each of the large

diamond crystal grains. Each crystal grain is com-

posed of several fan-shaped growth sections that

radiate outward from the centres of the circles. It is

easy to speculate that these large diamond crystal

grains initially nucleate as a spherical cluster and then

grow in three dimensions outward until coalescence

occurs with the adjacent diamond crystal grains.

Two types of distinctive structural features were

observed at the interface in specimen III as shown in

Fig. 6. The TEM image in Fig. 6a was taken at low

magni®cation and shows the overall structural fea-

tures. In the central area of this image, some crystal

grains with fan-shaped contrast can be observed.

However, in peripheral areas the crystal grains have

been masked by some round-shaped clusters or

blisters. The details of the clusters or blisters are

illustrated in Fig. 6b which was taken from the

peripheral areas at an increased magni®cation. Even

though these features have an amorphous-like appear-

ance, electron diffraction analysis, however, showed

that these clusters were polycrystalline diamond. It

can be inferred therefore that these nanoscale blisters

or clusters are diamond phase.

Fig. 6c and Fig. 6d show typical TEM micrographs

taken from the majority of the areas in specimen III.

From these images, the diamond crystal grains and the

fan-shaped growth sections within the crystal grains

can be seen more clearly. These growth features

observed by TEM for diamond crystals close to the

interface are very similar to those that have been

observed by AFM. The difference is that in the AFM

observations, the image contrast was caused by the

surface roughness, while here in the TEM observa-

tions, the image contrast is produced by structural

defects.

From these TEM images it is proposed that

diamond phase material initially nucleates randomly

on the substrate surface forming tiny clusters. In the

Fig. 6 (a±d). TEM micrographstaken from specimen III showingthe interface microstructures of anepitaxial CVD diamond ®lm


subsequent deposition process, these clusters develop

into large ball-like features by the continuous addition

of the deposited diamond component to these clusters.

With increase in the size of these ball-like features,

crystal facets gradually develop in certain favoured

directions. Because there is no con®nement of the

diamond crystal growth before coalescence, a high

density of defects will be accommodated within the

crystal grains. This is indeed true as all the electron

diffraction patterns taken from the crystal grain show

similar polycrystalline diamond diffraction patterns.

All of the diffraction rings can be identi®ed as

originating from diamond cubic lattice planes and no

other phases can be identi®ed.

Here, it should be emphasised that a prominent

intermediate layer has not been observed in this ®lm

although some cluster or blister-like features have

been observed on detached ®lms near to the interface.

There are some substantial morphological differences

between the diamond ®lm specimens III and II. The

crystal grains in specimen II are randomly oriented

while, the crystal grains in specimen III are well

aligned with each other showing typical epitaxial

growth features. From these considerations it seems

that a thick intermediate diamond phase carbon layer

does not exist in specimen III. It is highly improbable

that epitaxial diamond grains could grow on a

polycrystalline intermediate layer. Previous work

[12] has demonstrated the role of � silicon carbide

at the interface of diamond and silicon substrate.

Conclusions

In the TEM studies discussed here, some plan-view

TEM specimens have been prepared by ion thinning

only from the ®lm surface side to enable investigation

of the microstructural properties of the ®lm near to the

interface. The overall in-plan structural features of the

interfacial layer existing between the silicon and

diamond crystal grains was directly observed by this

plan-view TEM investigation. The polycrystaline

interfacial layer has been unambiguously identi®ed

as diamond phase carbon by both electron diffraction

and electron energy loss spectroscopy. These observa-

tions have con®rmed that the diamond crystal grains

in a diamond ®lm are not in direct contact with the

silicon substrate but instead form and grow on this

diamond phase carbon intermediate layer.

For the CVD diamond ®lm with epitaxial growth

features, no obvious interfacial intermediate layer was

observed. Epitaxial growth of the CVD diamond ®lm

required diamond crystals to nucleate directly and

grow on the substrate surface or on an epitaxial

interface layer which has a small lattice mis®t to both

the substrate and the thin ®lm material. Obviously, if

the interfacial layer is amorphous or polycrystalline in

nature, a crystallographic relation cannot be expected

between the diamond ®lm and the substrate. It can be

concluded that the elimination of the amorphous-like

interfacial layer at the earliest stage of deposition is

crucial to the heteroepitaxial growth of CVD diamond

®lms.

Acknowledgement. One of the authors (A. G. Fitzgerald) would liketo express his thanks to the Carnegie Trust for the ®nancial supportto enable him to carry out TEM experiments at the National Centrefor Electron Microscopy in the Ernest Orlando Lawrence BerkeleyNational Laboratory at the University of California, Berkeley.

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