Diamond growth by hot-filament chemical vapor deposition: state of the art

Diamond and Related Materials, 2 (1993) [277 1294

1277

Diamond growth by hot-filament chemical vapor deposition: state of the art* R. Haubner and B. Lux 1nstitute fi~r Chemical Technology of Inorganic Materials, Technical University Vienna, A-1060 Vienna (Austria)

(Received July 12, 1992; accepted in final form January 4, 1993)

Abstract Low-pressure diamond deposition using hot-filament gas activation was the first method to achieve nucleation and continuous diamond growth on various substrates. The method itself is simple but the strongly interdependent parameters involved must be strictly controlled in order to obtain reproducible diamond quality. The material used for the hot filament and its reactions during diamond deposition are important for gas activation. Together with this the chemical stability of the substrate and the deposition parameters determine the quality of the diamond layers produced. The growth conditions must be optimized for each specific application. When scaling up the hot-filament method, a uniform temperature distribution and the gas flow become additional factors of major importance for obtaining good quality diamond coating. The C : O : H ratio in the reaction gas also influences diamond growth rates and the resulting quality of the diamond films.

1. Introduction

1.1. History of the hot-filament method The first diamond nucleation on substrates other than diamond by low-pressure chemical vapor deposition (CVD) was produced in a hot-filament reactor [1]. The hot-filament method is one of the most common methods since it produces diamond easily and the equipment needed (Fig. l) is less expensive than with most other methods, except for the flame method [2]. Many other alternatives have been developed in recent years, such as those using electrical discharge plasmas, microwave, r.f. and d.c. glow discharges or jets [3]. Hirose and Terasawa [-4] reported that diamond films can be deposited onto silicon substrates at higher growth rates (8 10 ~tm h -a) by using organic compounds with high methyl concentrations, such as acetone, ethanol, methanol, trimethylamine, etc., instead of methane. Chen et al. [5] showed that the addition of oxygen-containing gases such as CO, CO2, 02 and H 2 0 to CH4-H2 could favorably affect the diamond crystallization and growth rates. The diamond deposition conditions, the reactor, the substrates and also some CVD diamond products have been optimized by many research teams during recent years.

*Paper presented at Diamond 1992, Heidelberg, August 31 September 4, 1992.

0925 9635/93/$6.00

1.2. Mechanisms of gas activation and diamond deposition using hot-filament deposition Diamond growth mechanisms under low-pressure conditions are not fully understood yet. A number of different models are under discussion [6, 7]. The importance of atomic hydrogen (H) and of CH radicals, as well as the effect of oxygen and OH radicals on the quality of the deposited diamond, were recognized very early [8, 9]. The in situ measurement of atomic H is rather complicated and not always possible. Atomic hydrogen has been measured by multiphoton ionization (MPI) spectroscopy at different filament temperatures

Gas inlet

I

~

,

~

~

hot filament (2000°C)

~

, L,.,,~,,,,~.~_)~_.~ diamond coating 11000°01 ~ j _ ~ substrate ......

,oex=

Fig. 1. Principle of the hot-filament low-pressure diamond synthesis.

,(-'~:1993

Elsevier Sequoia. All rights reserved

R. Haubner, B. Lux / Hot-filament CVD of diamond

1278

and CH4 concentrations [10]. Recently a simple method based on the heat produced during the recombination of atomic H was developed [11]. Thermodynamic calculations can give information regarding the stability of various radicals and other species at the gas activation temperature (filament) or at the deposition temperature (substrate surface) [12]. Since a large fraction of CH4 is converted to C2H2, a quasithermodynamic equilibrium of hydrocarbon species seems to be established while the gas mixture passes over the hot filament. From such measurements it is however difficult to judge the importance of specific species responsible for atomic attachment and the resulting crystal growth [13]. The formation and recombination of atomic H has also been calculated for different deposition conditions [14]. These results offer possibilities for optimizing certain experimental parameters such as gas activation, filament-substrate distance, CH 4 concentration, gas pressure, etc. Also the extent of diffusion of atomic H is of great importance for the diamond deposition process [15]. Experimental results indicated that free and forced convection effects are relatively unimportant for the quality of the diamond film and its deposition rate. Atomic gas diffusion is significant for the transport of important species surrounding the hot filament to the substrate surface (Fig. 2) [16]. In addition to the dissociation of molecular hydrogen to atomic H , the high filament temperature leads to cracking of the carbohydrides and converts part of the methane into acetylene, which is the thermodynamically stable compound at the filament temperatures. This process leads to a depletion of methane and methyl radicals in the vicinity of the filament and results in inhomogeneous diamond layer thicknesses [17]. A theoretical model optimized by Piekarczyk and Yarbrough [18] for the activation provoked by thermal methods considers four stages: (1) dissociation of the original hydrocarbon-hydrogen gas mixture at the dissociation temperature; (2) transport to the substrate surface, associated with a rapid cooling of the dissociated gas mixture; (3) simultaneous deposition of sp 3 (diamond) and s p 2 (graphite) hybridized carbon at the deposition temperature; (4) destruction by gasification or hydrogenation of sp2-bonded carbon codeposited in the preceding stage. Hot Filament

oonflguratlon 1 0.37

pm/h

Sample

oonfiguratlon 2

oonflguration 3

oonflguratlon 4

0 . 4 0 ~am/h

0.46 pm/h

0.48 ~m/h

Fig. 2. Diamond growth rates in various flow configurations [16].

From this model the following conclusions can be drawn. (1) Diamond will not be deposited from a gas phase thermally activated to a "low" degree or at least the deposition rate will be very low. (2) The diamond deposition rate should increase, and the concentration of graphitic (or other non-diamond) inclusions decrease, with increasing degree of activation (at constant concentration). (3) The deposition rate should increase with increasing transport rate of activated gases. (4) The deposition rate, and concentrations of graphitic inclusions, should increase with carbon concentration in the gas phase (at constant degree of activation). These conclusions agree quite well with the experimental results reported in the literature [18]. Tsuda et al. El9] assumed that methyl radicals and atomic H' play an important role during the deposition of diamond crystals. Organic compounds, which can easily generate methyl radicals, could therefore be preferable carbon sources during diamond deposition [4]. This opinion was also supported by Harris et al. [20] and more recent experimental results, while according to Frenklach's theories acetylene or species derived from it are considered to be the key compounds for the atomic carbon attachment kinetics occurring during diamond growth [21]. In view of the possible conversion of methane into acetylene during gas activation, as described above, this controversial matter seems to be of relatively little practical importance for the determination of the experimental parameters.

2. The hot-filament equipment For optimization of the hot-filament technique, everything connected with gas activation by a hot surface, thermal radiation from the filament to the substrate, and the deposition conditions must be properly controlled in order to deposit high-quality diamond. The parameters influencing the gas activation are not only the filament material (before and after carburization), filament temperature and filament geometry, but also the corresponding filament-substrate distance; all these influence the amount of activated particles reaching the surface and must therefore be optimized. Filament temperatures higher than 2000 °C are generally used because the effectiveness of the gas activation increases with increasing filament temperature. However, this also increases the amount of filament material included in the diamond formed as impurities. Thus, filament materials with very high melting points themselves (Re) or whose carbides have extremely high melting points (W,Ta) are used [22]. As the differences in the chemical natures and the physical properties of the

R. Haubner. B. Lux / Hot-filament CVD of diamond

filament materials have a direct and important influence on the gas activation and decomposition, diamond deposition is thus varied given otherwise constant deposition parameters [22].

(a)

2.1. Scaling up and types of reactors

(b)

In small laboratory reactors with only one filament and one substrate (Fig. 3(a)) [22, 23] the substrate temperature can be easily controlled by an external furnace. Diamond coating of a 3 inch Si wafer in a special hotfilament reactor constructed with one filament, whereby the wafer was moved slowly under the filament, proved to be an efficient way of enlarging the coated surface for very thin films (Fig. 3(b)) [24]. However, for scaling up the reactor and making thicker coatings, more filaments have to be used and regulation of the substrate surface temperature becomes a more complicated problem. The filament geometry must be optimized to achieve uniform thermal radiation and maximal gas activation (atomic H production) [14, 17]. Generally there are three types of basic reactor design (Fig. 4). The simplest type is a planar design with one plain horizontal filament and the substrates beneath [-25]. Much better energy efficiency is attained if substrates can be placed on both sides of the filament plane. In this symmetrical version a vertical arrangement of the filament and the two substrate planes seems to be better [26]. In this situation the gas flow has to be optimized to obtain homogeneous diamond depositions.

l

(a)

j.

Jy

/

gas inlet

/

quartz tube filament

_------ electrode substrates ~ - ~ s u b s t r a t e holder containing heater or cooler

(b)fil~ment

~

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substrate ,~Siwafer)

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¢ substrate

)<

heater

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| |} !

holder

containing

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~v

) "7"

subst~ate holder containing heater or cooler Fig. 4. Types of construction for scaling up a hot-filament reactor. If geometrically complex substrate shapes such as tools have to be coated [27], filament planes and substrate planes can be alternated. In this arrangement the substrate surface temperature must be controlled either by a sophisticated cooling system or by precise balance of the heat radiation of the filament to achieve the exact optimal substrate surface temperature, which is based on mathematical modeling [28]. 2.2. The hot filament as source for gas activation and thermal radiation

The filament materials, Ta, W or Re or the carbides of Ta and W, influence the deposition process via changes in gas activation and thermal radiation. The total hot-filament surface determines both the catalytic surface area actively involved in the formation of atomic H' and the radiating surface modifying the energy balance in the reactor. Electrical bias activation of the gas phase between the filament and substrate can be an additional parameter for controlling diamond deposition. Impurities as well as too high a carbon concentration in the gas phase can deactivate the filament surface, leading to a breakdown of the diamond deposition reaction. The substrate surface temperature is one of the most important deposition parameters. As the variation of almost any deposition parameter results in a change in surface temperature, calculation of the temperature and atomic H distributions allows geometrical as well as chemical optimization of the process. 2.2.1. Filament carburization

Fig. 3. Schematicdiagram of hot-filamentequipment for (a) static [22, 23] and (b) dynamic [24] diamond deposition.

The filament carburization process proceeds according to the following sequence of elementary steps [29]:

1280

R. Haubner, B. Lux / Hot-filament CVD of diamond

transport of CH4 to the metal surface and adsorption; decomposition of CH4 and chemisorption of C and H atoms; liberation of H , H2 and CHx into the gas phase; transformation of the adsorbed C atoms into the dissolved state; diffusion of C atoms in the metal lattice to form carbides; diffusion of carbon through the carbide lattice to form more carbide at the carbide-metal interface. Detailed information on the carburization rates of the different filaments, their geometrical changes, mechanical stability and changes in filament surface temperature during carburization as well as lifetimes and behavior during cooling and heating are required to produce the best design for a large-scale hot-filament CVD apparatus and to optimize diamond deposition and production. The carburization rate of various filaments (Ta, W) can easily be measured by means of their electrical resistivity, if the temperature is constant. Metallographic studies of cross-sections of the filaments allow a correlation between the resistance and the stage of carburization (thickness of the different carbide layers). With W and Ta filaments, carbides (MC) and subcarbides (M2C) are always formed. Re as a filament material shows no carbide formation, but recrystallizes [30]. All filament materials are embrittled duril~g use and show a tendency to form more or less s~vere surface cracks. The following can generally be observed [30]: Increasing the filament temperature increases the carburization rate and the thickness of the intermediate subcarbide layer formed (Figs. 5, 6). The resistivity data for the phases formed during carburization of the filament material are Ta 12.5 ~tf~cm, TazC 80 p.l) cm, TaC 25 I.tf~cm [31]. The overall filament resistivity changes permanently during the deposition. A maximum in resistivity for Ta filaments is observed at the maximum subcarbide layer thickness (Fig. 7). The carburization rate increases with increasing CH4 concentration and decreases with increasing gas pressure. The carburization rate of a filament also determines its deformation and crack formation. W filaments carburize faster than Ta filaments [30]; as a result W filaments deform more mechanically than Ta filaments [32]. Using W filaments for diamond deposition at 2400 °C is impossible because of the severe filament deformation and a high rate of W evaporation. Owing to carbide formation, Ta and W filaments can only be used for a limited number of heating and cooling cycles and their vulnerability to breakage during sample handling increases with the time and number of heating and cooling cycles. Since Re does not form a metal carbide, carbon has no effect on the filament geometry, but Re filaments recrystallize. However, they maintain their geometrical shape longer. For large-area depositions Re and Ta filaments yield

5h

24 h

U 0 0 0

o

0

m

I-

U 0 0 ~t

o

iU

E*o o

Fig. 5. Cross-sectional structure of Ta and W filaments at different filament temperatures (0.5% CH4, I0 Torr) [30].

500

only carbide phase

E

::3.

_...,,,_

400 O~

300

~200 (9

.'2_ 1 0 0 oo

i

i 20

I

I I I 40 60 Carburization time [hrs]

Fig. 6. Carburization rate of Ta and W filaments at 10 Torr [30].

R. Haubner, B. Lux / Hot:filament CVD qf diamond

1.5

E t-

DC -- DC negative (bias) (filament)

DC -- DC positive (bias) (filament)

0.5% CH4 10 torr 30 I/h

2 4 0 0 ° C

O

1281

~

DC -- AC (bias) (filament)

•

r.-

----.....,

~1.0

L - I + ~

2000"C

E

LL 0.5

....

I''''1 5

AC -- DC (bias) (filament)

.... I''''1 .... 10 15 20 C a r b u r i z a t i o n time [hrs]

I'''' 25

30

Fig. 7. Change in Ta filament resistance vs. time for carburization at 10 Torr, 0.5% CH4 [30].

more uniform films than W owing to their higher mechanical stability [22, 30]. The economical aspects depend largely on the relative lifetimes of Ta and Re. Re is much more expensive than Ta, but can certainly be used longer. However, realistic studies of lifetimes and performances of Re and Ta filaments under comparable diamond deposition conditions are not yet available in the literature.

2.2.2. Application of bias currents There are three different ways of enhancing gas activation using a bias voltage compared with the conventional hot-filament CVD without bias (Fig. 8) [33, 34]: (1) electron-assisted CVD (EA-CVD), (2) plasma-activated hot-filament CVD, and (3) d.c.-discharge CVD [34]. For all bias activation methods the experimental setup is quite important, since the different configurations for bias application using a.c. or d.c. sources for filament heating and for bias lead to different results (Fig. 9) [35]. When a d.c. voltage is applied between the filament

Fig. 9. Schematic diagrams of possible configurations for applying bias [-35].

and the substrate, the emitted current flows from the filament to the substrate. At high filament temperatures a gas discharge is obtained, causing an increase in the emitted electron current [33, 36]. The condition of the filament surface is important for the electron emission (emitted electron current). When the surface is contaminated with oxide or gas films, the electron equation may not be fully applicable, and the current density obviously must change [37, 38]. Precarburized filaments emit fewer electrons than pure metal filaments, and the electron current emitted decreases with increasing methane concentration (Fig. 10). The following general observations were made for a.c. current filament heating when a positive d.c. bias was applied to the substrate, i.e. EA-CVD. The bias application was studied in pure H 2 and C H 4 - H 2 using Ta filaments [33]. (1) The electron current emitted is increased by the Jbllowing: increasing the filament temperature, the filament surface area, the gas flow rate (marginally), and the applied d.c. voltage; decreasing the filament-substrate distance, and the gas pressure.

too TA-CVD

-

HOT FILAMENT CVD

" BIAS CVD ELECTRO~q ASSISTED CVD IEA CVD)

VOLTAGE DROP substrate - C ~ h o d e / f i l a m e n t

AC -- AC (bias) (filament)

DC'~ ~ T IVATED f-ILAMENTCVO

,_.p , ~ ~~ , - / -1.0~,~ u. • %40

PA-CVD DC D4SCHARGE CMD

80- .

~

x

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~

0

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60-

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> - -

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~, 150

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[V]

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300

400

Torr

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>2000

BURNING PLASMA

Fig. 8. Comparison of set-up and working parameters of hot-filament CVD, bias CVD and d.c.-discharge CVD [34].

500

1000

1500

2000

2500

Current [mA] Fig. 10. Influenceof CH4 concentration on the bias current at 10 and 40 Torr gas pressures (Ta filament, 2000°C, 10 mm, 30 I h 1) [33].

R. Haubner, B. Lux / Hot-filament CVD of diamond

1282

(2) Ta filaments emit more electrons than W filaments [39]. (3) Precarburized filaments emit fewer electrons than the uncarburized filaments (Ta 13%, W 45% decrease). (4) The optimal discharge (plasma power, intensity) finally established depended on the above parameters and the applied voltage. A change in substrate surface temperature is always associated with any bias application. Since an increase in the substrate temperature strongly influences the diamond growth rate [23, 33], this secondary effect must be considered when judging the overall effects of bias application [35].

filament power is not increased immediately to dissolve the carbon film, it will grow thicker and thicker owing to the decrease in the filament temperature [40]. By measuring the spectral emissivity of the filaments (W and Re) it was shown that the filament activity is highest if the filament surface is totally free of carbon [41]. The changes in filament emissivity, resistance, power consumption and CH4-C2H2 partial pressures have been attributed to transitions between deposition and etching of carbon occurring at the filament surface (Fig. 12(a)). The effects of oxygen on the filament activity were also studied. It could be shown that any oxygen added to the C H a - H 2 mixtures shifts the region where carbon deposits onto the filament towards lower temperatures. A significant improvement in filament activity was observed when oxygen removed carbon from the filament surface by the formation of CO (Fig. 12(b)) [42, 43].

2.2.3. Filament activity for gas activation

The filament activity and its influence on the deposition process depend strongly on two deposition parameters, the carbon content in the gas phase and the filament temperature. If the filament temperature falls below a critical value (depending on the carbon concentration) a carbon film forms around the filament (Fig. 11) [40]. This film deactivates the filament surface and the production of atomic H is strongly curtailed. A secondary effect is a decrease in the filament temperature due to increased filament resistance (at constant filament power). If the

2.2.4. Calculations of the formation of atomic hydrogen and the temperature distributions on the substrate To optimize the substrate surface temperature, the

distribution of thermal radiation from the filament over a given deposition area must be calculated. Figure 13 shows the distribution of substrate surface temperature for different substrate-filament distances. The decrease in surface temperature with increasing substrate-filament distance is shown in Fig. 14 [44]. Such calculations must be done with different filament geometries, diameters and temperatures to find the optimal temperature distributions and deposition conditions [17, 44]. These calculations are important mainly for solving scale-up problems encountered when designing large reactors. The formation of atomic hydrogen is affected by the surface temperature of the filament and the gas pressure. Thermodynamic calculations show that high temperatures and low pressures maximize the atomic H" formed

TaC graphite

Fig. 11. Ta hot filament coated by a carbon layer during diamond deposition [40]. 3000

........ '

.......

'

no c o n d o n s e d phase ~

2000

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/H=

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%.*.v,******* i ' m

lo6d . . . . i•6o . . . . . ia66 . . . . 2~'o0' " (b)

filament temperature PC]

Fig. 12. (a) Predicted phase boundaries for graphite in the C-H and C - H - O phase diagrams (25 Torr) [42]. (b) Spectral emissivities of initially carbon-covered Re filaments in H2 and in 1.5% C2H2 in H2 at 25 Tort [41].

R. Haubner, B. Lux / Hot-filament CVD o( diamond .

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2.5r

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(

X

2.5r

~

2000 5r

)

Fig. [3. Correlation between the substrate surface temperature distribution and the substrate-filament distance [44] (Try,=2500 K, r is the

I

2500 3000 3500 activation temperature [K]

4000

Fig. 15. Influence of gas pressure and temperature on atomic H formation [44].

filament radius, A is the substrate-filament distancet.

$ 2500

~filament

l~emperature

!

J J

E 2000 0

1500 E 1000 EE

0

10 20 30 40 50 filament/substrate distance [mml

Fig. 14. Influence of filament temperature and filament-substrate distance on medium substrate surface temperatures [44] (3 m m filament diameter, 10 m m filament length).

(Fig. 15) [44]. Calculations of H recombination are much more difficult because an essential parameter, interference with solid surfaces, is largely unknown. Thus calculations can only show general recombination trends under different conditions. Homogeneous recombination increases with increasing pressure and the H concentration at the filament decreases (Fig. 16(a)) [44]. Recombination can however also occur very efficiently on a solid surface (substrate or reactor). Its temperature is as important as its chemical nature and its roughness. For a given reactor wall and diameter, recombination at its surface is highest at low pressures and decreases with increasing pressure (Fig. 16(b)) [44]. Thus the reactor size is important for the degree of recombination and the resulting diamond formation (Fig. 16(c)) [44].

2.3. Measurements of substrate surface temperature in hot-filament systems The substrate surface temperature is one of the main parameters in diamond deposition. One problem in low-

pressure diamond deposition is its precise measurement and monitoring during deposition. There is always a temperature gradient between the surface (heated by radiation from the filament) and the substrate bulk, which is influenced mainly by the heat conductivity of the substrate and the filament temperature [23]. On an experimental scale it is possible to measure the temperature very close to the surface and study the influence of different deposition parameters [22, 23, 33]. The information obtained from such studies is summarized as follows• The substrate is heated primarily by radiant heat from the filament (especially effective at higher filament temperatures) and the recombination of atomic H' at the substrate surfaces. Convective heat transfer by the hot gas is less important. W filaments radiate more heat than Ta filaments• Consequently more electrical power is required for the W filament to attain a given filament temperature and the corresponding equilibrium substrate surface temperature is higher (Fig. 17). The substrate temperatures (bulk and surface) increase with increasing filament temperature (increased radiant heat). At higher filament temperatures the degree of H2 dissociation increases and more hydrogen atoms recombine at the substrate surface, increasing the substrate temperature (Fig. 17) [-23]. Reducing the filament-substrate distance increases the radiated heat reaching the substrate as well as the recombination of atomic H on the substrate surface. Both effects increase the substrate temperature [23, 40]. The temperature gradient across a single substrate with a single filament decreases with increasing illament-substrate distance as the radiant heat is more uniformly distributed over each substrate. At very short filament-substrate distances the radiated heat is concen-

1284

R. Haubner, B. Lux / Hot-filament CVD of diamond

3000°C filament tern )erature

100

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: "1-

.o_ E

10

..

10 torr

,

,.°.~ ....

° ....

°,

~ .°o.....o,...°o

°.o.q°o.

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0

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1

~......... ' '~.........: " ......... ! ........

0.1

0.01

o

'"

lo

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(a)

2o

3o

so

.o

time of recombination [ms] 3000°C

1001° ° , . . , °'. , ~ i. . . . . . . . .

~"

i i

°

filament ten" )erature ! . . . . . . q. ° ., . ° . . , ° ° , ,I, o = . , ° , ° ,¢, , , . o . . , °

~

!

•

5!cm

.......... )......... i ......... i ..........

...........

'

H in the gas phase leading to a reduced recombination effect at the substrate surface [44]. At higher gas f l o w rates, more electrical power is needed to keep the filament at constant temperature. The heat conductivity of the gas and higher rate of atomic H recombination on the substrate result in a marginally increased substrate temperature (Fig. 17) [23]. The thermal properties of the gas mixture also affect the substrate temperature. In pure Ar gas, lower substrate temperatures were measured than in pure H2 gas. H2 is a better conductor than Ar (Ta filaments not carburized). In Ar there is no heat transfer by recombination on the substrate surface. Lower substrate temperatures than in pure H2 with an uncarburized filament are also observed in C H 4 - H 2 atmosphere. The TaC surface has a higher emission than Ta metal. Also, there is greater recombination of atomic H directly in the gas phase containing CH4 rather than on the substrate surface (Fig. 17) [-23].

i ......... i ......... 3.

0.011 ~

\ ' V F u !

0

10

(b)

m _ 20

i . _ 30

-- i ~ 40

50

time of recombination [ms] lOO

~

2000°C filament temperature . . . . . . . . . I ~ - " i . . . . I d= diarn~terof the : , , , 10 t o r r I

lO

reacti,bn chamber tcml

........

.........

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i .........

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Diamond

deposition

Diamond deposition using the hot-filament method is possible under a wide range of deposition conditions. This makes comparisons of literature data and parameters which influence the diamond deposition difficult. However, many diamond nucleation and growth phenomena are similar to those observed with other deposition methods. They are discussed elsewhere [45]. The following sections summarize deposition results using the hot-filament method only, describing specific effects of deposition parameters valid for this method, such as filament temperature, filament-substrate distance, filament materials and bias investigations. 3.1. General comments

0

(c)

10

20

30

40

50

time of recombination [ms]

Fig. 16. Influence of deposition parameters on the lifetime of atomic H in the reactor [44]; (a) homogeneousrecombinationat 3000°C and various gas pressures;(b) heterogeneousrecombinationat 3000°C and various gas pressures (reactor diameter 50mm); (c) heterogeneous recombination at 2000°C and various reactor diameters (10 Torr). trated at the center of the substrate, with the area closest to the filament being hotter and the substrate borders being cooler [40]. The gas pressure affects both the substrate and the filament temperatures (Fig. 17) [23]. With increasing gas pressure, the substrate temperature decreases owing to a higher gas transport, heat transfer back to the gas phase and a more pronounced recombination of atomic

A major problem for all deposition experiments and descriptions of measured effects is the strong tendency of the different experimental parameters to interact. The variation of only one deposition parameter in most cases leads to the simultaneous change of several other parameters and conditions [23, 45]. Optimum conditions for growing coatings with diamond crystals of well faceted quality, good adhesion, and uniform coverage of a large area, all at an acceptable growth rate, can only be obtained by carefully considering and combining all available deposition parameters. However, in actual practice compromises in the choice of deposition parameters are always necessary to obtain specific results. 3.1.1. Diamond nucleation and nucleation enhancement

Many experiments have been carried out to study the effects of diamond nucleation on selected substrates [45].

R. Haubner, B. Lux ,' Hot-filament CVD o[ diamond

~'1000 pure

~' 1000 i 2200°C

H2

1285

constant:30 I/h, 10tort

800 600

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2oot

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constant: 30 I/h, 2200 C, 10, torr .... , .... , .... . .

10

20

30

40

~"1000

800 i600 400

(c)

10

20

40

constant:2000 C, 10 torr

800

5 torr

30

filament/sub=Irate distance[mm]

I/h, 2000 C

60 I/h

600

400 200

~• 200 ,,a .= 0

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~1000

constant: 3 0

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.

,

10 20 30 40 filamenlltubslratedistance [mm] i

looo

+oo

•

0

•

(d)

.....

, .... 10

, .... 20

, .... 30

, • , 40

filament/tub$traledistance [mm]

constant:30 I/h, 2200C, 10torr]

-..

'[

2ooi no gas ~Ar / 0 ...................... 0 10 20 30 40 (e) filamenll=ubstrate dizlance [mm] Fig. 17. Substrate surface temperature in relation to other diamond deposition parameters [23].

Diamond nucleation is very sensitive to the substrate material itself as well as to its surface condition and its preparation prior to deposition. An extensive treatment would be too lengthy here, so only a few literature references are given [46 51]. 3.1.2. Diamond growth

After a full-cover diamond layer is formed on a substrate, continuous steady-state growth conditions can be observed for constant deposition parameters. In this stage the deposition is usually no longer influenced by either the substrate properties or the original nucleation density [40]. As with all other diamond deposition methods, diamond growth is influenced by the gas composition, the surface temperature and the plasma intensity. Many deposition experiments have been run using different kinds of equipment which are not always comparable [45, 52]. Exceptions also valid for other methods are influences by elements either diffusing from the substrate through the deposited layer to the growing interface [53] or transported from the substrate through the gas phase owing to their high vapor pressures [54].

3.2. The diamond deposition parameters 3.2.1. Filament temperature

An increase in filament temperature results mostly in an increase in the substrate surface temperature because of higher heat radiation and higher atomic H' recombination (see Section 2.2.4). With increasing filament temperatures increasing growth rates are observed (Fig. 18(a)) [32]. If the filament temperature is decreased too much the deposit becomes ballas-type diamond. Increasing the filament temperature generally allows an increase in CH4 concentration (up to 5%) with well faceted diamond growth rates of about 8 lam h -1 (Fig. 18(b)) [32]. The influence of oxygen is not discussed here. Experiments using Mo wire nets showed that an increase in filament temperature results in a decrease in diamond nucleation density but leads to larger diamond crystals [14]. 3.2.2. Filament substrate distance

The filament substrate distance influences the homogeneity of diamond deposition. If a single filament is used the diamond deposit is best faceted directly below the filament, and with increasing distance the deposit becomes more ballas-like (Fig. 19) [40]. This is linked with the changes in heat radiation and the decrease in

1286

R. Haubner, B. Lux / Hot-filament CVD of diamond 12

/.'

6

,

filament temperat ou~*

/

//°

"0

~6

"8 4

8G)

8

C

.o_

#_ 0 (a)

~2600°C

t-~ o J I I

° I

L

I

I

i

I

2000 2400 2800 Filament temperature [°C]

._o I-

(b)

2oo0oc ~.0.--~--e--

O0

2

I

i

I

4 6 8 OH4 concentration [%]

J

10

Fig. 18. Effect of (a) filament temperature and (b) CH4 concentration on mean thickness of diamond film deposited for 1 h. Points designated with full dots signify films containing large amounts of graphitic carbon [32].

) IJrn 16 h

SiAION substrate

--Ta

hot filament

Fig. 19. Diamond morphology distribution under a hot filament [40].

atomic H concentration with increasing filamentsubstrate-surface distances. Several Mo nets placed over each other simulated a substrate surface temperature gradient and changed the substrata-filament distance gradient simultaneously. The diamond nucleation density and diamond grain size clearly decrease with increasing filament substrate distances (Fig. 20) [14].

3.2.3. Comparison of filament materials during diamond deposition Different research teams have almost always used only one filament material (W or Ta or Re) [55-57]. Thus very little data are available for comparing them. The filament materials influence the deposition conditions because of how their individual chemical and physical characteristics behave in the CH4-H 2 mixture

used for diamond deposition (carbide layer formation) (see Section 2.2.1). The maximum CH 4 concentration at which the filament is still active is also important [22] and varies with the filament material. By properly matching the two parameters methane concentration and filament temperature, it was possible to grow good quality diamond films at growth rates of more than 1 ~tm h-1 with all three filament materials Re, Ta and W [22]. However, different filament materials require different CH4 concentrations for maximal growth rates and the formation of well faceted, good quality diamond films (Fig. 21) [22]: 2200 °C Ta, 1.0%, C H 4 , 3.5 mg in 8 h Re, 2.0%, CH4, 7 mg in 8 h W, 2.0%, CH4, 10.5 mg in 8 h 2400 °C Ta, ~<3 % , C H 4 , 12 mg in 8 h Re, ~<2%, CH4, 12 mg in 8 h For Ta and Re filaments, increasing the filament temperature to 2400 °C led to an increase in the deposition rate if the optimal methane concentration at the higher temperature was used (Fig. 21). W filaments are not suitable for applications at or above 2400°C owing to severe deformation during carburization [22]. The general influence of the gas pressure, gas flow rate and filament temperature on the deposition of diamond are similar for all three filament materials, Ta, W and Re [22].

3.2.4. Influence of the applied bias voltage on diamond deposition As described in Section 2.2.2, diamond deposition under bias conditions is influenced mainly by changes in the substrate surface temperature and the specific experimental set-up. For an otherwise constant set-up with a varying substrate surface temperature in the diamond deposition experiments, it was shown that under zero bias and reverse bias (-150 V, - 5 gA) the diamond depositions

R. Haubner, B. Lux / Hot-filament CVDQfdiamond

1287

p/Torr

H 10 p,m Fig. 20. D i a m o n d deposition onto stacked molybdenum wire nets (Tr...... =700 C , Tf~=2000 °C, 0.5% CH 4 in H2) [14].

are similar, and well faceted crystals are formed. Under forward bias (+150 V, 125 mA and d.c. discharge) the film quality is poorer [58]. Similar results were also obtained by other research groups [59, 60]. Using reverse bias, diamond nucleation and growth was detected by a distinct change in electrical resistance between the filament and the Si or Mo substrates [61]. If the substrate surface temperature is held constant during all bias experiments and compared with a nonbias experiment with the same substrate surface temperature, the following results are obtained [33]. The applied d.c. voltage shows no significant influence on the nucleation rate of the diamond crystals at shorter deposition times. With an applied d.c. voltage better faceted diamond crystals are obtained at higher pressures (40 Torr) without raising the substrate surface temperature than without bias. For a longer deposition time (16 h) bias results in an increased growth rate of the diamond coating.

Deposition results using W filaments are generally comparable with those obtained using Ta filaments.

3.2.5. Other deposition parameters investigated by the hot-filament technique The gas pressure influences the gas activation, mainly through the formation of atomic H' and the gas speed in the reactor. If the pressure is too low (less than 5 Torr) the gas speed increases dramatically and the gas no longer contains enough atomic H for the formation of well faceted diamond deposits. If the pressure is too high (above 100 Torr) the recombination of atomic H' in the gas phase before reaching the substrate can become so high that again ballas deposition occurs [62]. 02 addition to the gas phase is an important parameter which can be used in hot-filament CVD to grow diamonds at higher CH 4 concentrations, with higher growth rates and better quality [63]. The large increase in growth rate reported (up to 10~tm h -1) if acetone is

R. Haubner, B. Lux / Hot-filament CVD of diamond

1288

Ta filament

W filament

Re filament

@

must be run at constant substrate surface temperatures. The optimal diamond deposition growth rate was found at a substrate surface temperature of 810°C [23].

u

4. Characterization of hot-filament diamond

e

o~

15

I/h

t

~)

. .i

30

I/h

60

I/h !

I

lO,um Fig. 21. Influence of CH4 on diamond deposition at 2200 °C using Ta, W and Re filaments (8 h, 10 mm, 20 Torr, 30 1 h - 1) [22].

used instead of methane [4] could not be observed under similar deposition conditions [64]. Deposition onto hard metals. Because of the industrial importance of coated tools [65], many deposition experiments were run on hard metals using hot-filament CVD. The Co content in this substrate material must be taken into consideration when choosing the deposition conditions in order to minimize the tendency of Co to favor the deposition of amorphous carbon at the substrate-coating interface [66, 67]. Also Co diffusion on the substrate surface and, under certain conditions, through the diamond layer, is clearly revealed by the formation of Co precipitates at higher deposition temperatures [68, 69]. Diamond deposition onto hard metals is in principle similar to that on other substrates (Si, SiA1ON, etc.) but because of the different chemical nature of the hard metal the deposition conditions needed to obtain diamond coatings suitable for wear applications are also different [66, 70]. It has been observed [70] that the diamond films deposited onto hard metal substrates at relatively low substrate surface temperatures (760___10°C) show improved adhesion. This can be explained by reduced interfacial stress [40]. To measure the real influence of deposition parameters and obtain significant results, the deposition experiments

Raman spectroscopy was one of the first methods used to differentiate between diamond and amorphous carbon deposits [71]. The amount of amorphous carbon in the diamond layers and the layer purity can be easily detected by Raman spectroscopy. In well faceted diamond layers amorphous carbon is found in the grain boundaries. Because of this the diamond grain size and the amount of grain boundaries and their thickness strongly influence the quality of the diamond deposited [32, 72]. By increasing the amount of grain boundaries (smaller crystal size) the amount of impurities increases. Raman analyses are thus most important in the case of fine-grained, thin diamond layers, where diamond crystal facets are not readily visible. Another possibility for quality control is scanning electron microscopy (SEM) [73]. With this method the crystal habits of the grown diamonds can be determined (octahedral (111) or cubic (100) facets). The study of diamond morphology allows interpretations of the deposition process. Cubic (100) facets are for the most part formed if the diamond is deposited at lower temperatures and lower gas activation [74]. Because of the high heat radiation during hot-filament deposition, mostly octahedral (111) crystals are observed. Only under special conditions is it possible to grow diamond with cubic (100) facets by hot-filament CVD. Changes in diamond growth produced by impurities can also be detected easily by SEM (Fig. 22). Transmission electron microscopy (TEM) allows the characterization of small internal crystal defects, dislocations, diamond grain boundaries and atomic resolution of substrate-coating interfaces [75]. Such methods are especially important for characterizing layers for optical and electronic applications. High-resolution pictures of hot-filament diamond reveal the following [76]. There are many defects in CVD diamond layers, and their amount increases if the CH4 concentration in the reaction gas increases (smaller diamond grain size); in well faceted diamond crystals grown at low CH4 concentrations the defects consist mainly of twins and stacking faults, in some places highly disordered regions exist at the cross point of twin boundaries; no phases other than diamond are detected in these hot-filament diamond deposits. The characterization of diamond layers deposited under different bias conditions shows that the defect density in the diamonds is highest if forward bias, resulting in a d.c.-plasma discharge, is used. At no bias

1289

R. Haubner, B. Lux / Hot-filament CVD o( diamond

1

'

x) um

'

lo um

Fig. 22. Diamond deposits showing different morphologies.

or under reverse bias conditions the defects in the diamond are similar but their density is lower [75]. The formation of textured diamond coatings can be observed in hot-filament CVD just as with other CVD deposition methods [45, 77]. If the ratio between crystal density and layer thickness is high enough, textured films with only cubic (100) facets on the surface can be grown under special conditions [78]. Stress measurements by X-ray diffraction indicate that for specific deposition conditions and special substrates diamond coatings with low internal stress can be produced [79]. The hydrogen content in diamond deposits is apparently correlated with the amorphous carbon present in the grain boundaries of diamond layers [80]. The amorphous carbon contains higher amounts of hydrogen because the free valence electrons are saturated by the hydrogen in the sp 2 bonds. In agreement with Raman observations, there is also a correlation between the diamond grain size and the hydrogen content in the diamond layer. Secondary ion mass spectroscopy (SIMS) measurements of hydrogen in a well faceted diamond layer showed 0.2 at.% H and in a ballas-type layer 1.2 at.% H [40]. By analysing the hydrogen content with the nuclear reaction method using 15N, it was possible to determine a clear correlation between the hydrogen content in the layers and the diamond morphology. The hydrogen content increases from well faceted diamond layers with large crystals (approximately 0.2 at.% H), to well faceted diamond layers with small grain sizes (approximately 0.8at.% H), to ballas-type diamond layers (approximately 1.4 at.% H). Because of the good depth resolution, clear separation between the high hydrogen coverage on the surface and the nearly homogeneous low hydrogen content in the bulk was possible [81]. Metal impurities can be measured in diamond layers by Rutherford backscattering (RBS). They result through evaporation of the filament metal and are homogeneously distributed in the diamond layers [82]. W depth profiles measured through the diamond layers by SIMS

showed higher W concentrations in the film substrate interface region [83]. This can be explained by the higher W evaporation at the beginning of the deposition process when the filament is metallic. After carburization of the filament surface the evaporation decreases. Boron impurities from a BN source in the reactor showed a homogeneous concentration profile over the layer thickness [83]. The BN was etched by the atomic H present in the gas phase and contaminated the deposited layer. The electrical properties of diamond are of major interest for applications such as heat sinks or semiconducting materials. Measurements showed that the resistance can be increased by an annealing process. A comparison of diamond deposited by hot-filament with microwave and by the d.c.-plasma method showed that the resistivity of hot-filament layers is higher before annealing and lower afterwards [84]. n- and p-type diamond layers have been deposited by phosphorus (ntype) and boron (p-type) doping. The phosphorus-doped layer showed a resistivity of l f~ cm and the borondoped layer 0.1 f~ cm. A combination of these two layers allows the production of a p-n junction diode [85]. lsotopically pure diamonds for maximal thermal conductivity were grown with 12C or 13C carbon sources. The thermal conductivity is influenced by the isotopic composition and reaches a maximum in isotopically pure diamonds. Isotopically pure CVD diamond was used to grow high-pressure isotopically pure gems [86]. The thermal conductivity of diamond is utilized in electronics in heatsinks which are produced from larger diamond sheets. These sheets are metallized with a Ti Pt Au multilayer and placed in a laser diode array between a copper cooling block and the diode. The light output increases owing to better cooling when a diamond heatsink is used [87]. Hot-filament CVD is however not recommended for this application. The filaments, especially W at temperatures above 2000 °C, contaminate the diamond by evaporation, so that the maximum heat conductivities cannot be reached [88]. Optically smooth, very thin (100 nm) diamond films have been deposited by increasing the nucleation density

1290

R. Haubner, B. Lux / Hot-filament CVD of diamond

to 101° cm -2. The transparency and smoothness of the resulting d i a m o n d films could be demonstrated by measuring reflection spectra which showed strong interference oscillations [24]. The oxidation rate of C V D d i a m o n d films at high temperatures does not depend strongly on the structure of the films. The films synthesized by the hot-filament m e t h o d are somewhat more oxidation resistant than the d i a m o n d films deposited by microwave C V D [89].

into account. Critical evaluations of the raw materials and the energy required as well as the cost of investments are necessary. An exact assessment demands a t h o r o u g h knowledge of the processes, but such information is still scarce in the literature. Bigelow and Henderson pointed out the importance of the specific energy per carat deposited, c o m p a r i n g arc jet and microwave C V D [90]. It is too early to give a full evaluation, but typical data taken from the literature are summarized in Table 1 [62, 91-96]. Cost efficiency will necessarily play a key role in the selection of the best scaling-up m e t h o d and will decisively influence the further developments in this technology. The main advantages of the hot-filament m e t h o d are as follows. The equipment is inexpensive, i.e. reactor and gas activation source (filament and filament heating source). Large deposition areas of h o m o g e n e o u s deposition are possible with multiple-filament arrangements. Deposition conditions can be easily controlled by variation of the gas activation parameters (filament parameters). The calculated electrical specific yield (kilowatt hours per gram) for d i a m o n d seems to be relatively high for

5. Comparison of the hot-filament method with other methods Every m e t h o d used for industrial d i a m o n d deposition must fulfill the technical as well as economical requirements. Apparently certain sophisticated, unique d i a m o n d products such as thin membranes can be sold at very high prices per carat [65]. However, most wear products will have to be available at m u c h lower costs in order to be competitive. Despite the large n u m b e r of methods available, the choices for industrial production methods are limited when economical and technical requirements are taken

TABLE 1. Survey of parameters applied for the most common methods of diamond deposition Parameter

CVD method Hot filament

EA-CVD

d.c.discharge

Arc dischargejet

r.f. induction

Microwave

Electrical power (kW) Chemical power (kW) Gas input (%CH4) H2 flow (sccm) CH4 flow (sccm) C2H2 flow (sccm) 02 flow (sccm) Ar flow (sccm) Deposition rate (l~m h- 1) Deposition area (cm2) C supplied (mg h-1) C deposited (mg h-1) CCE (%) Electrical specific yield (kWh g-1)b Chemical yield (kWh g-1)d Pressure (mbar) Surface temperature (°C) Substrate

0.3

?

1.6

9.0

60.0

1.5a

0.96 478 4.64

0.5-2.0 5-50 0.25-1.0

2.0 98.0 2.0

0.75 1500 60

1.0-10.0 0-12000 100-1200

2.5 800 20.0

Reference

Combined flame 0.33

975 1025

1.3 8 139 3.5 2.5 428

-4.0 0.25 7.5-30.0 0.352 1.2-4.7 ?

-250 0.25 60.0 22 37 73

6500 400 3.1 1800 436 24 21

8000-35000 I00 3.1 3000-36000 65-164 0.2-5.0 370-920

-2.2 15 600 12 2 267c

100 1 58000 9.86 0.017

20-50 10005 SiAION

53 700-900 Si, SiC, Mo, WC

266 950 Si, SiC, AlzO3, Mo W, diamond

250 1000 Mo

1000 700-1200 Mo

100 1000 SiA1ON

33 1000 - 1200 Si, Mo, TiC, Ta, BN, Cu

62

91

92

93

94

95

96

sccm, standard cm3 min-1; CCE, carbon conversion efficiency. a 1.5 kW microwave and 1.7 kW supporting furnace. bElectrical specific yield, related to electrical power input. CRelated to 3.2 kW (1.5 kW microwave and 1.7 kW supporting furnace). alChemical specific yield, related to chemical power input (C2H2 + 0 2 ~ C O + H20).

1291

R. Haubner. B. Lux / Hot-filament CVD q( diamond

hot-filament deposition, but there are no energy losses during energy transformation for plasma regeneration (r.f., microwave). The main disadvantages of the hot-filament method are as follows. The filament materials have a limited lifetime. The diamonds deposited contain traces of the filament material. The reproducibility might be lower than with commercial microwave deposition units. For high reproducibility automatically controlled commercial units would be needed.

b] diamond slmtheeisof CVD filament

[b)lamer~tting ray ) laser,,mo.d

S II~_~trat( ~X~US--~-~-'-J t feed

~trate

,CVDdiamond

:)razing ~ ' , ~ . . ~

2% ie|OVDcnamondTAB toole

hSnk

6. Potential products and industrial applications Based on known principles, research and development efforts currently being undertaken by a number of important companies have already led to new products. For manufacturing diamond products in situ, CVD diamond deposition or "freestanding layer" bonding can be used (Fig. 23). The wear and cutting applications utilize mainly the superhard properties of diamond. The production of technically and economically feasible products for superhard applications is still only in its early stages. The rapid advancement in these "simpler" application areas will most certainly continue and even gain momentum. Some of the products developed using hot-filament CVD are listed below. Optical and electrical applications are in preparation; all these developments are at a very early stage.

~ ~

~-substrate

id)brazing

l

|o)uw Made outting

I

II

1 ---[~j~r~_substrateJ ~

Ioldilllm,lutionof lubstrate

~k~

acid

laser~ay CVDdim~l~°c~l) ~ Oiu

kSfrleestandingCVO diamondpieoel

[~

|l~ CVD dlam~KI ~tli~l t~)lll

6.1. In situ CVD diamond deposition (CVD coating analogy)

Coating of PCD tools can significantly improve their cutting performance and lifetime [97]. Twist-drills with diamond coatings can be used to perforate difficult materials such as non-ferrous alloys (A1-Si), green ceramics, hard metals and reinforced plastics. Figure 24 shows different cutting edges [98]. TAB (tape automated bonding) stamps are used in microelectronics to connect chips to their wires and circuits. In this application the high thermal conductivity of diamond as well as the high wear resistivity are utilized. In tests the TAB tools coated with CVD diamond showed excellent results compared with those containing single-crystal diamond. Besides the economic advantages, the low-pressure diamond layers can further offer the possibility of producing much larger tools than can be done with single-crystal high-pressure diamond [99]. Metal-cutting tool inserts for turning and milling of hypereutectic A1-Si alloys are also now a preferred

[ endmill ]

brazing~

~ .

[ insert ]

cemented carbide

Fig. 23. Production process of an in situ CVD diamond-coatedTAB tool and of free-standing CVD diamond sheets brazed to cutting tools [99]. application recently investigated by a number of research groups [3, 99, 100-106]. 6.2. "Bonded" freestanding diamond sheets ( PCD analogy)

Products produced by the freestanding layer technique are of great interest because of their relatively simple production method. Freestanding layers have no Co or other binders in the diamond sheet which is an additional advantage over PCD products. Evaluations of shearing tests with polycrystalline CVD

1292

R. Haubner, B. Lux / Hot-filament CVD of diamond

uncoated

diamond coated

|II II II

J O ,tl

4~ q~ a

Fig. 24. Cutting edges of cemented carbide twist-drills before and after testing and comparison between uncoated and diamond-coated inserts [98].

diamond films, hard metals and PCD tools showed that CVD diamond has excellent tool lifetime (four times longer than PCD; 100 times longer than hard metal alone) [107]. Figure 23 shows a typical endmill with a bonded freestanding diamond layer [99, 108]. Small-diameter endmill cutting performance during machining of copper-coated printed circuit boards (GFRP) reveals the clear superiority of bonded freestanding CVD layers over hard metal alone [99]. It must be pointed out that the diamond products mentioned here were all produced by the hot-filament CVD method. These and the products developed by the other deposition methods share the common goal of achieving the ultimate in performance [65].

7. Conclusions and outlook The hot-filament diamond deposition process is relatively easy to handle and construction of the reactor is both simple and inexpensive. Filament materials, their carburization and the arrangement in the reactor are well known and easy to reproduce. Diamond deposition is possible under a wide range of well known deposition conditions (filament temperature, filament surface, substrate surface temperature, gas pressure, gas flow rate, methane concentration, substrate material, applied bias, etc.). Since diamond films produced by the hot-filament method contain impurities from the filament, they are used mainly in wear applications.

Because the hot-filament method is very easy to scale up to industrial dimensions this method is one of the most important for the future.

Acknowledgments We wish to express our gratitude to the Austrian "Fonds zur F6rderung der wissenschaftlichen Forschung" which has been sponsoring work at the Technical University in Vienna (Projects P6031, P7274). These projects are being carried out within the framework of the international "D-A-CH" "German-AustriaSwiss" cooperation. Thanks go also to those in the management of Sandvik Stockholm, who believed in these ideas long ago and through the years have sponsored various parts of our research work. Dr. Staffan S6derberg provided helpful discussions prior to and during the hot-filament research work. Last but not least, the authors wish to thank all members of the research team at the Technical University in Vienna, who have devoted their time and energy to solving successfully the many challenging problems encountered in this work.

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43

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