Low Pressure Synthesis of Diamond Coatings

Keynote Papers

Low Pressure Synthesis of Diamond Coatings H. E. Hintermann (1) and A. K. Chattopadhyay, CSEM SA, 2, rue Breguet, 2007 Neuchbtel (Switzerland)

SUMMARY

In part 1 of this paper a concise resume is given on the present understanding of the reigning mechanisms of CVD low pressure diamond synthesis, in particular of nucleation and early growth. The formation and involvement of atomic hydrogen and hydrocarbon radicals is essential for growing diamond films; they are the determining active and interactive species. In part 2 the potential of CVD diamond in mechanical, especially tool applications, is treated. A state-of-the-art overview of current industrial uses, capabilities and limitations of CVD diamond products is presented and discussed. Part 3 deals with an entirely new application of CVD diamond with definite industrial potential, i.e. with thin film diamond sensors. The feasibility of such devices has been shown; nevertheress, to make the product commercially viable, depends on the successful development of an industrially economic process technology - hence, presents a strong CIRP related challenge. KEYWORDS: Surfaces, Diamond Coatings, Manufacturing Processes Acknowledgement Contributions made by the following CIRP members and Tool Manufacturing industries as well as members of the scientific and technical staff of CSEM towards the preparation of this keynote paper are gratefully acknowledged. Universities a n d Research Organizations

-

J.P. Kruth, Katholieke Universiteit Leuven, Leuven, Belgium H.K. Tanshoff, J. Thalemann and R. Seidel, Hannover Technical University, Hannover, Germany A. Korhonen, Helsinki University of Technology, Helsinki, Finland T. Moriwaki, Kobe University;Kobe, Japan A. Grisel and coworkers, MICROSENS SA, Neuchltel, Switzerland W. Hanni, P. Alers, V. Neuman, (Miss) M. Ruegg, (Mrs.) D. Buri, I. BBguin, M. Ziiger, CSEM SA, Neuchltel, Switzerland.

Tool Manufacturing Industries

-

H. Wapler, De Beers Industrial Diamond Division, Ascot, England D. Bhat, GTE Valenite Corp., Troy, USA Dennis T. Quinto, A. Inspektor and C.E. Bauer, Kennametal Inc., Latrobe, USA R. Porat, A. Layyous, ISCAR LTD, Nahariya, Israel P.M. Stephan, Norton Company, Diamond Film, Northboro, USA W. Schintlmeistei, Plansee Tizit GmbH, Reutte, Austria C. Stjemberg, Stellram SA, Nyon, Switzerland K. Shibuki, Toshiba Tungaloy Co., Kawasaki, Japan

1. 1.1.

LOW PRESSURE DIAMOND SYNTHESIS, "CVD DIAMOND" Introduction

It can safely be stated that the determining steps towards low pressure diamond synthesis by Chemical Vapour Deposition, CVD, and towards the development of thin coherent polycrystalline diamond coatings, had successfully been achieved by scientists of the former Soviet Union as early as the late 5 0 s and during the 60s, among them Derjaguin and Spitsyn. Though in the US some activities of the same kind went on, the Soviet developments stayed unobserved for a long time. Only the 80's brought about the revelation and potential of the low pressure diamond synthesis, first in Japan, then in the US, then in Europe, all based on the earlier findings of the Soviet scientists.

1.2.

Chemical vapour phase deposition (CVD) of diamond

The chemical reaction is seemingly simple. A hydrocarbon component is decomposed at high temperature and in the presence of hydrogen in a plasma. Many reaction schemes have been proposed, none so far is unambiguously proven. What is generally agreed upon is that the formation of atomic hydrogen (He) and hydrocar-

Annals of the CIRP Vol. 42/2/1993

bon radicals R - C H p , e.g. methyl radical CH3.. and the interaction of these species with the C-surface radicals, i.e. active carbon sites at the diamond surface, = C* , is essential (1.2). It may very well be that under the action of the plasma other, higher, radicals emanating from repeated stripping of hydrogen atoms from the hydrocarbon molecules are involved in the accumulation of added C-atoms at the growth sites of the diamond lattice, namely:

. . .- $.

( H2C ,H q *,

* ) , radicals derived from methane, CH4.

or ( H C = C -) and ( .C = C .) , radicals derived from

acetylene, C2H2, respectively. The global reaction, when using a reaction mixture e.g. of methane and hydrogen, can be written according to equation [I]. T m4 (g) ___._) c d (s) + 2H2 (g)

plasma achvated

[I1

or, in steps of activation according to eq. [2] and [3], eq. [4] being the sum of reactions [2] and [3] T "2 H2

m4+H*

plasma activas

H.

< T > Cl-lg*+H:H + H2 plasma activated

+

CH~*+H~ (~4+1/2~2 plasma achvated

141

The hydrocarbon active species, such as typically the radicals CH3= or . C = C . , (or any other of the aforementioned hydrocarbon radicals), and H- are the determining reactive species. These react on a diamond surface, covered by adsorbed or chemisorbed hydrogen (Fig. 1) to form active carbon sites (sp3). The mechanisms involved could comprise a first step, where the atomic hydrogen (He) reacts with the hydrogen bonded to the diamond lattice to form recombined H2, leaving behind, momentarily, at lattice sites of the diamond surface, an unsaturated, active carbon with a dangling bond. In a second step, this dangling bond is occupied by one of the active carbon containing species derived from a hydrocarbon mentioned earlier, thus adding another carbon atom (or other carbon atoms) to the diamond growth surface. This is illustrated in the reaction scheme according to equations [5] and [61:

769

H

H

H

H

which has a slightly lower enthalpy (about 2 kJ/mole) than diamond, to CO and C02, and thus eliminates this type of contamination from the sites diamond should grow. When diamond is contaminated with the graphite allotrope, this graphite is mainly found in the grain boundaries of the polycrystalline diamond deposit.

H

1.3. Deposition modes Among several CVD related methods for diamond deposition, three of them, with technical potential, are shortly depicted here for illustration and better understanding: 1.3.1. Thermal CVD (Hot filament, TF-CVD) , Fig. 2 The plasma, required for the formation of atomic hydrogen and hydrocarbon radicals is generated by a hot filament (W, Ta, Re,...), heated to 2000°C or higher, in the neighbourhood of which Fig. 1: Dangling bonds at the surface of a diamond lattice occupied by hydrogen A similar scheme could be drawn for using acetylene as a precursor for activated radicals, ( H C I C .) or ( C I C .) .

.

Atomic hydrogen, (He), can react simultaneously at more than one -H site of the methyl group, (CH3-), attached to the diamond surface, and, hence, early growth at a nucleation site of the surface formed in this way, expands in several directions, forming first clusters, then nanocrystals, then microcrystals. On growing together, a coherent polycrystalline film and coating is obtained. If the process is conducted at a heterosubstrate, e.g. on a cemented carbide (cc), polycrystalline films can be tolerated; for active electronic components, such as ICs, however, epitaxially grown films, e.g. on silicon, are needed. Polycrystalline films are more or less contaminated, depending on the type of process applied and the purity of the gases used; polycrystalline film deposition is state of the art at many places. Extended epitaxial growth and three dimensional growth of diamond single crystals, however, are still in their infant state of development. C

C

I I

+

C-C:H

H*

C hydrogen saturated dangling bond of diamond surface

T I + C-Cc.

plasma activated

+

I

H:H+

H2

C

diamond growth site recombination with dangling bond, of hydrogen ready to react with a hydrocarbonradical to add one further C to the diamond lamce

atomic hydrogen

[51

C

I C-C* I

C H

I 1 I I

T

+CH3*

C

C-C:C-H

plasma activated

C-

C-

C-

C*+ H2

1 1 1 c H .H

Y

Vacuumpump

Fig. 2: Scheme of a hot filament CVD reactor hydrogen is dissociated into highly reactive atomic hydrogen and hydrocarbon compounds are stripped from a hydrogen (or several hydrogen atoms) forming a radical. In the simplest case, that of methane, CH4, a methyl radical, CH3*, or probably a HC-. radical, is formed. The substrate is between 5 and 20 mm distant from the heated filament; its temperature is situated between 700 and 1OOO"C. The gas composition is typically Hz:CH4=99:1. Under these conditions the growth rate is of the order of 1 pnfh. The filament, unfortunately, evaporates to a small extent and, hence, contaminates the growing diamond film. This metallic contamination is not too much of a constraint for coatings used in mechanical applications such as tools or general wear parts: however, it is a nuisance when envisaging electronic applications such as active components, as well as optical or sensor devices. 1.3.2. Microwave and electron cyclotron resonance CVD (pW/ECR-CVD), Fig. 3

C H

1

T*plasm +He activated

iiY

0 2' Substrate

+CH3*

+ CT,plasma

C

H

C

H

The plasma is generated in the reactive gas mixture by a high-frequency electric field, such as microwaves, or by electron cyclotron Turbomolecular pump

I I C- C*+ H2 I I

0 4" Si (100) substrate

J.. H.

--

-vacuum Pump

and so on

According to the type of process and the conditions of deposition the carbon atoms bind to the diamond growth sites either in the lattice of the the graphite allotrope of carbon, sp2, or in the lattice of the diamond allotrope, sp3, searched for. Carbon growth by sp2 bonding (graphite formation) is undesirable. Small amounts of oxygen added to the reaction gas mixture can be beneficial, in so far as it bums off the co-deposited graphite, sp2,

770

n Z + cn4 Fig. 3: Scheme of a microwave plasma activated CVD reactor

resonance (ECR), i.e. a combination of electric and magnetic fields. By using these methods, the coatings are very uniform (? 10 96 of average thickness), over large areas (200 mm and more), smooth, and of high purity. But, the growth rate is very low, of the order of 0.1 ~ m / h .By this process large areas of uniform, homogeneous, polycrystalline thin diamond films of high quality can be obtained, predestined for electronic, optical and sensor applications.

1.3.3. Combustion synthesis (oxy-acetylene torch and RF-high energy plasma torch, respectively), Figures 4-6 The plasma is generated either by a chemical flame, e.g. an oxygen-acetylene torch , 0 2 - C2H2, as depicted in Figures 4 and 5

(3,4,5), or by a DC or HF torch, respectively (Fig. 6). Both methods, chemical and physical, operate at very high temperatures, the chemical torch above 3000K and the plasma torches up to 8000K.In both cases the substrates to be coated need to be cooled. When using an oxygen-acetylenetorch the deposition mode using a flame in the turbulent rather than in the laminar flow regime presents some advantages especially in what concerns the growth rate, which can be an order of magnitude greater. In both modes the coatings are irregular, non homogeneous, but relatively pure, depending on the purity of the gases. Acetylene, C2H2, - unfortunately - cannot easily be cleaned economically to high purity. The growth rate can be orders of magnitude greater than obtained by other methods, and is, for an oxygen-acetylene torch in the turbulent flame regime, typically 50 pm/h (5). 1.4.

Diamond properties

A summary of important and interesting properties of CVD diamond is given in Table l. Table 2 compares thermal properties of diamond against other materials hitherto used in applications where heat management is of prime importance, such as e.g. in heat sink applications or for cutting tools.

Cu Substrate holder

t

'

I Water cwled Cu block

Fig. 4: Scheme of a combustion synthesis: set-up for oxygenacetylene torch deposition of diamond

BURNER

The quality, especially the structure and purity of CVD diamond, is determined by X-Ray analysis and Raman- and p-Raman spectroscopy. Reference is made to (6) where more literature citations on these analytical methods and the interpretation of the spectra can be found.

In essence, in Raman spectroscopy a laser beam (Ar or Heme) passes through the sample under investigation. Part of the light is scattered. The frequencies of the scattered light are different from that of the incident laser beam. The difference between the frequencies of scattered and incident light are a measure of the quanta of vibrational energy, induced by the diamond deposit giving rise to an emission peak, the so-called Raman-shift. This Raman-shift at 1332 cm-1 is the characteristic peak for sp3 bonding of the carbon allotrope diamond. A broad peak near 1499 cm-1 indicates graTable 1: Properties of CVD diamond

-

hperties

diamond

m

single crystal diarond'

3.51

3.515

2100 11 1OOoI

2200

INNER CONE

Density [g/cm3]

ACETYLENE FEATHER

'&rmalconductivity

at 25°C [w/mK] OUTER FLAME-

*

Thermal expansion LAMINAR

TURBULENT

Fig. 5: Flame shapes of oxygen-acetylene burners (laminar and turbulent, respectively ) Ar + H :!

+ CHq

-Plasma

torch

coefficient, range 25-2OOOC [x 1W6PCl

2.0

1.5 - 4.8

Bandgap [eV]

5.45

5.45

Index of refraction at 10 pn

2.34 - 2.42

2.40

Electrical resistivity [R.cm]

10'2- 10'6

10'6

Dielectric constant [45 MHz - 20 GHz]

5.6

5.70

Dielectric strength [V/crn]

106

106

Loss tangent [45 MHz - 20 GHz]

< o.Ooo1

Sanuated electron velocity [lo7 cm/s]

2.7

2.7

1350-1500 480

2200

Vickers hardness number [kg/m21

5000-1oooO

5700- lo400

Coefficient of friction [highly polished surface state]

0.05 - 0.15

0.05 - 0.15

u

O.OOol*

Canier mobilities

-

[cm2/ V.SI

- elecuon [n] - positive hole [p]

Mo tip Cu holder

TC

1600

Fig. 6: Scheme of a HF plasma torch CVD reactor

+

*

parallel respectively perpendicular to growth axis below currently measurable values

771

Table 2:

Heat management parameters, comparison between aluminium nitride [AN], beryllium oxide [BeO] and diamond [D]. ~~

Roperties

Be0

Diamond [D]

Resistivity [Q.cm]

1014

1014

1012- 1016

Dielecuic consrant

8.8

6.7

6.6

Dielectric smngth

105

105

106

140-220

260

c 17W>

0.7

1.o

0.51

4.1

1.2

2.0

at 1 MHz

Wlml T h d conductivity

Iwlfil Specific heat

3. Difficulties in making tool with in-built chip breaker and complex geometry 4. Size restriction - particularly difficult when making small diameter tools.

The above-mentioned limitations with polycrystalline diamond tools are considered to be the principal reasons for the current interest in R&D activity in CVD diamond for machining applications. Also, the cutting tool industry has been predicted by many to become one of the first-large-volume applications of CVD diamond (15). 2.2.

Diamond films by low pressure CVD (Chemical Vapour Deposition)

CVD diamond combines the advantage of both single diamond stone and PCD and can be stated as follows:

[JlgKl Thermal expansion, range: 0 - 400°C [x 10-6PCl

phite-like carbon impurities, sp2; another one, at 1590-1600 cm-1, indicates pure graphite, and still another peak at 1350 cm-1, amorphous carbon species. The laser irradiation also generates luminescence, in general broad peaks, the intensity and the shift of which are a measure of the degree of distortion and the structural faulting of the diamond lattice.

1. Binder free diamond coating with higher hardness, wear resistance, thermal resistance than PCD 2. Highly dense polycrystalline pure diamond coating without the inherent shortcoming of single crystal cleavage. However, it is contended that the good throwing power and ability to control properties,such as nucleation, grain size, adhesion with the substrate and surface smoothness of the film are critical to the application of the CVD technique for enhancement of the reliability and performance capabilities of tools (16). According to a tool manufacturer (17) consistent and predictable performance of a coated insert means: 1. Good bonding of the diamond layer to permit a predictable abrasive failure mode as opposed to flaking

2.

2.1.

APPLICATION OF CVD DIAMOND IN MECHANICAL MANUFACTURING Introduction

Diamond is the hardest of all known materials. It is available in natural form since long. Natural diamond tools are used mainly for special applications where no other tool can perform satisfactorily. High quality single crystal industrial diamonds are the only options for applications such as finish turning of gold, copper front surface mirror, microtome knives (7). On the other hand, lower quality industrial diamonds are extensively used for high speed machining of non-ferrous metals, ceramics, plastics. Other uses of single stone diamond are in surface-set drill heads in mining and oil-well applications, in machining cold pressed sintered carbide preforms and sometimes to shape stone, concrete and a dressing tool for abrasive wheel. It is also used as die stone in wire drawing operation (7,8). However, the anisotropy of such a "single stone" is well known and the unreliability of the tool because of easy cleavage has been well recognized (7-9). Moreover, limited supply, increasing demand and high cost have resulted in an intensive search for an alternative dependable source of diamond (7). This search led to the ultra-high pressure and temperature synthesis of diamond from graphite and the subsequent development of polycrystalline diamond tools (PCD) in the late 1960's (7). The polycrystalline diamond tools consist of a thin layer (0.5 to 1.5 mm) of fine-grain size, randomly oriented particles sintered with a binder phase (usually cobalt) and metallurgically bonded to a cemented carbide substrate. These tools are formed by a high pressurehigh temperature technique. The main advantage of PCD tools is the greater toughness (resulting from random orientations of diamond grains and the corresponding lack of simple cleavage planes). But, limitations of PCD tools were also well understood (9-14) and stated as follows: 1. Tooling cost is rather high 2. Presence of Co makes the tools less wear resistant and less thermally stable than single crystal

772

2. Adequate properties of the film, e.g. wear resistance, microhardness, edge coverage and thickness uniformity 3. Resultant workpiece surface finish within the limits set by the customer.

2.3.

Types of diamond films

2.3.1. Thick films

In this case the diamond film (>500 pm thick) can be grown on an "easy" substrate. Later, the diamond "sheet" can be brazed to the actual tool substrate and the primary substrate is removed by dissolving it or by other means. The thick film diamond Fmds its uses in making inserts, drills, reamers, end mills, routers (18,19). 2.3.2. Thin films The thin film technique is meant for direct deposition of pure diamond films ( 4 0 p n thick) directly onto the tool substrate. This is also believed to be the area where CVD diamond is expected to yield the greatest advantage over PCDs, including multiple edges and pressed-in chip breakers (15). 2.4.

Substrate for thin film diamond tool

Due to the success of cemented carbide in other machining applications and considering its toughness, wear resistance and resistance to plastic deformation (20). this material appears to be the best choice as the tool substrate (10,15). Another consideration which substantially influences the preference of cemented carbide is that the tool shape is very flexible since it can be formed more easily into the desirable shape than sintered diamond. However, it was soon discovered that Co, a good solvent for'carbon and hence a good carbon carrier, and the very ingredient which gives the carbide the essential toughness, not only inhibits adhesion of diamond to the carbide substrate but also aids the formation of non-diamond material including soot (15,21,22). Good diamond can only be expected on carbide grades with higher Co content for which the binder phase has been removed from the substrate surface (23,24).

A number of techniques have been proposed to remove Co from the substrate surface which include etching with an acid, electrolysis, vapor etching, heating under vacuum (18,24,25). Appropriate surface roughness of the carbide substrate is also necessary to obtain optimum film-substrate adhesion strength (24). Applications of various interlayers including a functionally gradient intermediate layer (26) have also been proposed to promote nucleation and adhesion of diamond film on cemented carbide, tool steel and stainless steel substrates (18,21,26,27). A heat treatment of the intermediate layer could also improve the adhesion of the diamond film to the cemented carbide substrate (28). The deposition of a diamond film, even on an unetched cemented carbide substrate containing Co, but with good adhesion has been made possible through the use of a low temperature deposition technique (29).

between diamond and carbide (33). Many tool manufacturers use this substrate in their thin film diamond coated tools (1 1,15,21,34,35,36,37). However, higher cost and lower toughness of ceramic compared to that of carbides can also be noted (21). One manufacturer recommends its ceramic coated thin film tools for the finishing operations only. They may not perform as well in high impact machining as their carbide counterparts (21). The use of sintered diamond with Co or S i c matrix as the substrate for diamond films has also been reported (38). A special grade of S i c substrate with extreme toughness and good compatibility with thin diamond film has also been used (39,40). The important design feature of such coated tools is that on the coated tool the edge strength and consistent film adhesion is maintained by the brushhoned edge of the substrate, yet the sharpness of the edge is regenerated by polishing the table and reducing the edge rounding effect as shown in Fig. 9 (39).

An improvement in the adhesion of a diamond film on a cemented

carbide substrate, particularly with a curved surface, has also been achieved by incorporating reinforcing phases between the cemented carbide substrate and the diamond films (30). A remarkable improvement in the adhesion strength of diamond film on Co-free WC substrate has been claimed by pre-decarburization of the substrate surface before diamond coating (31.32). The most significant effect of the treatment is considered to be mechanical reinforcement adhesion strength by the rugged interface derived from fine WC grains on the substrate surface (31,32). Indentation tests with Rockwell diamond at 60 kgf load showed flaking of the film around the indentation for the diamond film deposited on the untreated substrate. Diamond coatings on the predecarburized surface exhibited a higher adhesion strength with hardly any flaking as shown in Fig. 7 (a) and (b) (31). Scratching (load: 3 kgf) of the film on an untreated surface caused considerable flaking of the film, while the film on the decarbu-

Fig. 9: Scanning electron micrograph (SEM)of CVDITE showing polished table and sharper cutting edge (39) 2.5.

Cutting tool application

2.5.1. Thick films Thick film CVD diamond brazed to a WC + Co substrate has established an unquestionable supremacy over conventional cemented carbide tools in milling S i c reinforced aluminium composites (35) and carbon fibre reinforced composites (34). Sintered diamond coated with a diamond film also showed better flank and crater wear resistance than uncoated sintered diamond tool in turning hot pressed alumina (38,41). Fig. 7: Diamond films after indentation test: (a) untreated substrate; @) decarburized surface. Load: 60 kgf (31) rized substrate exhibits slight chipping at the edge of the scratch mark as shown in Fig. 8 (a) and (b) (31).

However, machining of AI-Si alloys is found to be one of the potential application areas for such tools. In longitudinal turning of Al-alloy (containing 12 % and 17 % Si) thick film diamond coated tools exhibited better wear resistance than PCD (13,14,34,42). This improved capability of coated diamond tools was reestablished when the Si content was as high as 22 % (12). This is shown in Fig. 10. Such tools also exhibited better wear resistance than a PCD in turning plastic containing hard particles (14) and alumina reinforced aluminium composites (35) and S i c reinforced Al-alloy (43). In ultra-precision turning of aluminium, sintered diamond tools could not match the thick film diamond tool in terms of surface finish of the workpiece (43).

Fig. 8: Diamond films after scratch test: (a) untreated substrate; (b) decarburized surface. Load: 3 kgf (31) An alternative approach to the problem of poor nucleation and adhesion of the diamond film is to use Si3N4 substrates. It is easier to diamond coat Si3N4 because it is Co-free. Additionally, the thermal expansion mismatches between diamond and Si3N4 is less than that

In the milling operation, thick film tools were superior to PCD in wear resistance while machining A1-12 % Si alloy (13,14). Sometimes 40 % less flank wear has. been observed over PCD in face milling of A1-17 % alloy (34). Small diameter (0 3 mm) two flute end mills made with 100 p thick CVD diamond plate and brazed to a cemented carbide shank showed less flank wear and better surface finish in comparison to a K10 carbide grade in milling A1-10 % Si alloy (13). Figure 11 shows that a similar tool with single flute and even smaller diameter (0 1.5 mm) had extreme wear resistance in comparison to K10

773

0.400

1

-0-

cemenled carbide (KIO)

-5-

C V O OIA.

-9- C O M P A X 1500

7

-*-

feed rate O~rnm/re~ 05mm depth of cut cutting speed ?OOm.'mir,

E

C O M P A X 1600

03

0

10 20

30 4 0 SO 6 0 7 0

80

90 100

C u t t i n g time ( m i d

Fig. 10: Relation of cutting time and flank wear (12) carbide grade in machining printed circuit-board. A carbide end mill caused more burr formation than a CVD diamond tool (1 3). It is also stated that a thick film tip had much longer life and more stable cutting behaviour than a thin film tool in turning AC4C alloy (9).

I

I

I

50

10G

15:)

Cutting time

(min)

Fig. 12: Flank wear during turning of an Al 20% Si alloy. The thickness of the diamond film is about 6 pm A Diamond coatings on untreated WC; 0 diamond coatings on decarburized WC (31)

2.5.2. Thin films The performance of thin film diamond coated cemented carbide tools could be influenced by the surface condition of the substrate which controls the adhesion strength. During dry turning of AI-20 % Si alloy the wear resistance of a diamond coating on a decarburized substrate was remarkably higher than that of a diamond coating on the untreated WC substrate as shown in Fig. 12 (31). Similarly, surface roughness of the coating could have a negative influence on the wear resistance of the coating which is submitted to mechanical impact of Si particles during turning of AI-18 % alloy. Under such conditions a 3 pm thick diamond coated tool showed 3 times more flank wear than the sintered diamond tool (25). The micro roughness of the coating in the form of pin-holes

However, in machining carbon fibre reinforced plastic (52 % CF), the rate of flank wear of a thin film diamond coated Si3N4 tool was as low as that of a PCD (37). A similar coated tool with WC + Co substrate could also equal the performance of a PCD tool in 'profiling, turning, boring and facing of fibre reinforced plastics (17). Machining of sintered Cu-alloy (Cu-I0 8 Sn) has been found to be an area, where thin film diamond coated tools undoubtedly surpassed their PCD counterparts. The t h h film tool exhibited a 10 times longer life in comparison to the sintered diamond tool. The pure diamond thin film reduced the wear promoted by chipping

Cemented C a r b i d e (Down C u t )

Cement

CVD Diamond (Down C u t )

I

Y

I

I

I

I

I

4

8

12

16

20

C u t t i n g Length (CI) Fig. 11: Wear resistance of single flute CVD diamond endmill (1 3) or outgrowth can increase the drag force of the chip and increase the risk of early failure of the too1 due to flaking of the coating as shown in Fig. 13 (15).

Fig. 13: Diamond coated tool edge of a) good and b) poor (outgrowths) quality. Rapid wear of edge in b) after c) 0.5 and d) 2 min cutting time (15)

a;DIA-COAT

Polishing of diamond film is very effective to prolong the life of a tool and to improve workpiece surface finish as shown in Fig. 14 (44). It reduces BUE (built-up edge) formation and ensures precision machining (9). The advantage of smoothness of dhmond films has also been reflected on the surface finish of the work piece (17). In milling and turning of an Al-alloy containing 16-18 % Si, thin film diamond coated tools having cemented carbide, binderless carbide, Si3N4 and a special grade S i c substrate, showed a shorter life compared to conventional PCD (1 5,17,29,34,39,44,45).

( AS-COATED) 0:DIA-COAT

(POLISHED)

'; SINTEKED

O t I O O '

'

'

'

500

*

"

CUTTING NUMBER [ P A S S ]

'

1000 I

'

DIAMOND

4 CEMENTED

CARBIDE

Fig. 14: The surface finish (Ra)of Al 10%Si alloy (44) because of the absence of any binder metal in the coating, which might have enhanced welding with the workmaterial in the case of PCD (10).

774

Thin film CVD diamond coated Si3N4 tool displayed remarkable wear resistance in comparison to uncoated Si3N4 tool in turning an A1-18 % alloy as shown in Fig. 15 (15). In similar application a 101 advantage could be found over a cemented carbide tool (34). In milling of A1-18 % too, a Si3N4 tool with a thin film of diamond very easily outperformed a micro-grain carbide (35). Diamond coated Si3N4 tools not only improved the surface finish of the workpiece (Al-12 % Si alloy) as shown in Fig. 16 (11). but the tool life was also enhanced by 20 times of that of a WC + Co grade tool. In machining carbon fibre reinforced plastic (CFRP) (52 % CF), a diamond coated Si3N4 tool showed much better wear resistance than a carbide tool as depicted in Fig. 17 (37). A thin film diamond coated ceramic end mill showed remarkable advantages for composite machining over carbide tools. A cleaner cut

Carbide

V=100 m/min

d=0.5 mm f = O . 1 am/rev Wet c u t t i n g

U .d

5 L (d 0,

50.1

X C (d

4

cr Flank

Q)

wear

a

(y) 0.5 0.4

-

.r(

v)

v: 300 mlrnin s: 0.1 rnmlrev a:1.5 rnm Dry

Q

-

0.3

-

02

-

Uncoated Si3N4

0

1000

50 0

Cutting length

in

Fig. 17: Wear progress curves of various tools in machining of 52 V O ~% - CFRP (37) higher life was possible with a diamond coated drill (02.1 mm) in drilling carbide green compact (47).

.. 0

50

100

* 200

150

250

Cutting time (min)

Fig. 15: Flank wear vs cutting time for uncoated and diamond coated silicon nitride tools in longitudinal turning test

A thin film diamond coated S i c tool was superior to K10 carbide grade in turning A1-18 Si alloy (39). The milling test further demonstrated that both, the S i c substrate and the CVD diamond layer, were extremely tough and sufficiently impact resistant. The coated tool offered 10 times the stock removal capability with respect to a carbide grade in turning Ti 6A14V alloy (40). Figure 20 (40) demonstrates the wear resistance of diamond coated tools in such an application.

(15)

-=I P Y.

25

0

Cemented carbide diamond -Diamond coated

---. Sinterd

SPGN120308

i

I

I

I

I

I

I

5

10

15

20

25

30

Cutting distance [ km 1

Fig. 16: Machined surface roughness of work materials using different cutting inserts (11) Fig. 18: Composite matenai macninea with tungsten carbide top) and diamond coated endmills (bottom) (36) could be made with a diamond coated tool against a frayed quality of cut using a WC tool as dem0nstiated.h Fig. 18 (36).

2.6. A thin film diamond coated cemented carbide tool was capable of machining A1 and Al-1 1 % Si alloy with less BUE and better surface finish than that obtained with a T i c coated insert (46). In turning A1-18.5 % Si alloy and semi-sintered Sic, the coated tool offered good wear resistance. The tool also qualified in light milling but in heavy milling of Al-11 % Si alloy, it failed mainly because of peeling of the coating (46). Diamond coated cemented carbide was superior to uncoated carbide in turning and milling of A1-12 % Si alloy (10). Figure 19 (10) shows the results of. a turning test. Diamond coated carbide tool could easily outperform Td coated carbides in graphite turning (17).

In drilling operations too, diamond coatings offer substantial advantages. Ten times longer life could be achieved with a diamond coated carbide drill (04.2 mm) in machining alumina green compact in comparison to uncoated carbide (9). Similarly, 25 times

Abrasive tool application

Usually diamond abrasive grits are obtained either by crushing natural diamond to different sizes or by synthesis from graphite by the high pressure and temperature technique. A preliminary study suggests that diamond products obtained by CVD may be a good candidate for abrasive machining of ceramic material (48). Furthermore, diamond-ceramic hybrid particles have been developed with diamond deposition of S i c resulting in thick diamond layers around S i c particles (49). It was also possible to produce superhard composite particles containing a CBN core’particle surrounded by a coating of CVD diamond (50). Such composite particles may be of interest for producing superhard particles used in abrasive machining. Nickel based alloy containing a certain quantitity of transition elements of the group VI B such as chromium can wet the surface of

775

The new technology may obtain industrial acceptance as a better means when the following requirements are met: 1. Ability to grow diamond abrasive grit with controlled friability 2. Ability to control grit size, protrusion and spacing

a

3. Ability to grow diamond crystals on substrates with complex geometry

$

3

4. Ability to prevent fall out of the grit from the substrate.

0.2

E

DC46 0.1

2.7.

10

20

30

Cutting time (min) Fig. 19: Wear rate diagram of DC46 and cemented carbide after turning test. (Work material: A1 12% Si, speed: 1000 m/min, depth of cut: 1 mm, feed: 0.1 mm/rev, dry cutting) (10) diamond rather easily without being treated (51). Wetting and bonding is promoted by the formation of a Cr-carbide rich reaction zone at the interface (52). Wetting of the bare surface of CBN particles with the same alloy is extremely difficult (5334). The new superhard composite particles with a CBN core and diamond coating may offer an effective solution to the above-mentioned bonding problem.

1 .o

-E -E s

0.6

It seems that a product oriented technology requires to be developed to achieve the above-mentioned objectives.

KC210

TEST PARAMETERS Test material: Ti6A14V Dimensions: 1Ocrn diameter Cutting speed: 100mImin Feed rate: O.2mrnlrev Depth of cut: 1 .Omm

Q

3

0.:

In the wire drawing operation, a CVD diamond coated die did not show any change of the wire diameter even after drawing 5 tons of copper. The die diameter was 0.412 mm, on the other hand diameter expansion was observed in a natural single crystal diamond die. Ring wear was found to form in the natural diamond die, while many streaks were found in the CVD diamond die. The streak formation has to be prevented with a thick diamond coating composed of finer grain and more close packed structure (12). Some success has also been reported in improving the performance of shearing tools which were made by using thermal filament CVD of diamond (55). The use of diamond films to form knife edges appears to be a real possibility (56). Such knives may replace natural diamond surgical and ophthalmological knives in the future. The use of CVD diamond films in wear parts is also increasing. Sensor anvils which always touch abrasive wheel surfaces to perceive wear have the twin requirements of high hardness and wear resistance. A thick CVD diamond film is being used in this application (12). Similarly, a diamond coated micrometer anvil resists wear in the toughest WC, ceramic and abrasive applications providing longer tool life and consistently precise and accurate measurements (57). CVD diamond is also getting wide application in lead bond tooling (12J3). The requirements for such tools are high thermal and wear resistance and excellent heat conductivity. The life of such a tool is defined by the number of times the tool can perfectly bond lead wires (Au, Al) to IC's (13). In this respect CVD diamond (100 pxn thick) showed longer life than PCD while the bonding cost is the same for CVD coated and single crystal tools (13).

2.8.

Cutting time [minl

Fig. 20: CVDITE vs coated carbide KC 210 tool life turning test (40) For efficient coating of abrasive particles having an irregular shape, a fluidization system has been used (49). Various methods have also been proposed in the patent literature (18). As a substitution for electroplating, CVD techn'iques are employed for producing various products including an abrasive sheet of metal coated with diamond particles, wire saws, marking pens, metal scissors and core drills (18). CVD has also been extended for the production of grinding wheels (18). It can be quite interesting for manufacturing abrasive tools with monolayer configuration. However, tailoring of the substrate surface is a primary consideration which may be substantially influenced by the type of material (e.g. steel, hardmetal, ceramic) and their composition. For slender tool, high stiffness of the substrate material is an important consideration for precision machi. ning. In this respect, hardmetals and high strength engineering ceramics would be the best options.

776

Other applications for special tools and general wear parts

Concluding remarks to CVD diamond applications in tools and general wear parts

The potential of CVD diamond films for machining applications is well recognized. Thick CVD diamond films brazed to a cemented carbide substrate have exhibited their superiority over uncoated carbide or even PCD in turning, milling and drilling composites and Al-alloys containing as much as 22 % Si. On the other hand, compared to PCD tools, the life of thin film diamond tools is limited by the thickness of the coating and its adhesion to the substrate. However, adhesion still continues to be the critical factor, particularly for carbide substrates. Although substantial progress has been made to impfove the adhesion, such coated tools still may face stiff competition from PCDs, particularly in those application areas where tools in the shape of indexable inserts may be used. Again, the limitations of PCDs are well known in the domain of rotary tools which is mainly dominated by cemented carbide. Thin film diamond coated tools can replace several rotary carbide tools because of their remarkable wear resistance. There are already indications that tool manufacturers are keen on using the CVD diamond technique to manufacture abrasive tools. Similarly, the possibility of diamond coatings for use in wire drawing dies, shearing tools, lead bonding tools, is also demonstra-

ted. The industrial use of CVD diamond films in wear parts has also increased. However, the choice of diamond coatings will be decided by both the requirement and the overall gain in economy for each application. 3. 3.1.

APPLICATION OF CVD DIAMOND IN ELECTRONICS AND MICROSENSORS Introduction

Diamond is the material of extreme properties. Since it has become feasible to deposit diamond as thin films of only a few atomic layers in thickness as well as thick coatings of up to 1 mm and more on substrates other than diamond, i.e. on industrial, functional substrate materials, many new applications can be envisaged to make use of diamond's extraordinary properties not only in mechanics (chapter 2), but also in optics, optoelectronics, microelectronics, bioengineering, and as sensor elements, advantageously integrated with its IC. Diamond produced by low pressure CVD (LPCVD) methods compares well in purity and in properties with high pressure, high temperature (HPHT) and natural diamond, respectively. As has been demonstrated in chapter 2, the hardness of diamond can be and will be exploited to make coated cutting tools, with increased lifetime, a higher production output, and featuring a largely improved quality of the machined surfaces. Diamond will provide protective and free standing windows or optical coatings with high transmittance over a wide range of wavelengths, reaching from the visible to the far infrared (FIR). Membranes for mechanical and acoustic applications represent another domain of present and future applications. Its high thermal conductivity, high resistivity and high breakdown voltage make diamond a strong candidate as a packaging material for electronic and sensor devices. In the highly polished state diamond is an excellent low frictiooigh wear resistant material with large potential for dry or emergency lubrication. The bandgap of diamond with 5.5 eV is very large, and so are its electron and hole mobility when doped, making it a promising semiconductor for high-temperature, high-speed power devices. Among these, doped, e.g. B-doped, diamond sensor devices have reached a development state which is close to production. However, just as high adhesion is considered to be the one determining property and prerequisit for diamond films on cutting tools, so is a high and controlled purity the prevalently required property for electronics and sensor applications. This does not mean that other properties such as uniformity, homogeneity of structure, epitaxy, thickness and surface roughness over large areas, high deposition rates and others are not important either: but without sufficient adhesion in the case of tools, inserts and general wear parts, or without high purity in electronic and sensor applications, the system will just not be feasible. 3.2.

CVD diamond-films in sensor devices

In this context the development and production of simple diamond based sensors in mass quantities will be considered. To make a diamond film based sensor in micron dimensions ever a success as an industrial product, it has to be shown, that such a sensor can technically and economically be produced in large numbers. Hence before the development of such a sensing device, advantageously integrated with its own electronic circuit, will be further developed, it has to be ascertained that not only the market will respond to such an item, but also that its production will not become hampered by technical or economic hurdles. Hence it is a typical case for what CIRP stands for: production engineering research. How can a sensor of a given type based on diamond films be produced in large quantities provided the deposition processes are mastered and the homogeneity and the quality of the coatings are acceptable? Today's production of most microsensor devices is based on silicon, hence it is an offspring of a standard, well mastered microelectronic technology. The deposition of diamond by different techniques on Si wafers of up to 8" diameter has been reported by various groups (58-64). The criterion is to which extent these dif-

ferent groups master the uniformity, homogeneity in structure and thickness of their coatings to allow their use at production scale. Uniform deposition of diamond thin films on Si by DCplasma technology and TF-CVD have been reported by (58,59), and by ECR microwave techniques on Si wafers of up to 8" diameter by (60-64). The uniformity of a blanket coating on a 8" Si-wafer is reported to be of the order o f f 10%. In order to be feasible for the production of thousands of sensor devices, the uniformity and the homogeneity, however, should be better than f 5% (62,65). To reach this goal is but a matter of time and effort. What has to be proven now and well in advance of further efforts to improve the quality of the coatings, i.e. before large sized uniform and homogeneous diamond coated wafers have been developed to satisfaction, is the feasibility of the production technologies. The envisaged production method by CSEM (62,65,66) for patterning a diamond coated Si-wafer is an adapted SAD, i.e. a selected area deposition process, derived from microelectronics production technology. What had to be proven is the possibility to accurately and reproducibly etch the diamond on silicon (Si) or silicon nitride (Si3Nq) for patterning by using a standard lift-off technique. To apply this technology, dense nucleation and pinhole free growth of diamond on Si- or Si3N4 surfaces must be achieved if homogeneous and uniform coatings of controllable structure and morphology (65-70) are to be obtained. .The commonly used method except (71) - of enhancing nucleation by scratching the Si surface with diamond powder is in fact useful, nevertheless the Si surface will be damaged by this technique and the nucleation density will not exceed l08/cm2. Hence, this method of activation is not compatible with and applicable to SAD. Rather a "soft" nucleation method has to be applied, which first had to be developed. The. method is proprietary. Using undamaged Si surfaces resulting from this new nucleation process, and working under clean room conditions, a novel method of SAD has .been developped for patterning diamond coatings by the standard lift-off technique used in microelectronic technology. The envisaged diamond based sensor devices on Si are thermistors, anemometers and pressure transducers (66,72,73). Standard polished p- or n-doped (100) Si wafers of 4" diameter were used. Often though not exclusively SigN4-films have been deposited on the Si wafer in thicknesses of 100 to 250 nm by low pressure CVD (LPCVD) prior to further processing. Si3N4 films are electrically insulating, avoiding thus the formation of any ohmic contact by tiny agglomerates of amorphous carbon or graphite co-deposited during the process, thus giving rise to diode formation between the p-doped diamond film (p-diamond) and the silicon substrate (65). If the wafer has been coated on both sides with Si3Nq the backside can be used as a mask for selective etching of the silicon if so needed. 3.3.

Selected area deposition, SAD

Three types of masking techniques for patterning have been developed (65).

3.3.1. Photoresist mask The first and most economic one for SAD diamond on Silicon and Si3N4 coated silicon is the lift-off technique practiced in ordinary microelectronic processing using a photoresist masking for SAD. Figure 21 shows the sequence of the operational steps for this technique. Figure 22 presents a patterned meander structure produced by this technique. Random growth of diamond offside the selected areas can be found on both types of substrates though to a lesser extent on Si3Nq than on silicon, if the photoresist is not properly applied and not pinhole-free. The surface topography of a patterned diamond coating deposited on 200 nm Si3N4 on silicon according to the lift-off technique using photoresit masking is shown in the SEM micrograph of Fig. 23. Pinholes in the photoresist are marked by undesired nucleation and growth of diamond at these sites. Random growth of diamond is found offside the selected areas, i.e. where it should not be. The adhesion of these diamond outgrowths is high, indeed; they cannot

777

1

#

Silimn Wafer

(100)

LPCVD Nitride Si3N4

Positive Photoresist PPtternimg Develop Photoresist

F

Diamond Pretreatment Strip Photoresist Mask

-

Silicon

Si&

-

-

Photor.sist Diamond

pDiamond

Fig. 23: SAD diamond structure on Si3N4 (200 nm)on Si with randomly nucleated diamond

Fig. 21: Sequence of operations of diamond patterning by the photoresist lift-off technique

3.3.3.

be removed by HF or BHF etching of the silicon surface in an ultrasonic bath. A fracture cross section of the Si/Si3N4/D (D for diamond) coating composite is shown in Fig. 24. The diamond outgrowths at the rim of the patterned structure do not make an electrical contact to the silicon substrate -just because of the interposed insulating bamer layer of Si3N4 in between. As opposed to this, by applying a diamond structure directly on a silicon substrate, diode formation cannot be avoided, and hence leakage currents will flow.

Thirdly SAD of p-diamond (electrically conductive) on blanket diamond (electrically non-conductive or, if contaminated, less conductive than p-diamond) is camed out also with an Si& mask. The oxide film is sputtered on the blanket diamond. The patterning of the oxide film is done again by a lift-off process. This and the entire sequence of processing steps is schematically depicted in Fig. 29. The procedure described is particularly useful for the production of free standing diamond membranes as well as patterned pdoped diamond on electrically non- or low-conductive blanket diamond foils.

3.3.2.

SAD p-diamond on blanket diamond

Silicon oxide mask

This SAD method uses Si02 masks of 60 to 1'000 nm thickness. By using this type of a mask the random growth of undesirable

A) Diamond/p-Diamond B) Si3N4 C) Silicon

Fig. 22: Developed photoresist film in the form of a meander structure on Si3N4 on Si by using the photoresist lift-off technique diamond offside the selected areas can be greatly reduced and even entirely avoided. Another advantage is, that it can easily be removed by a HF etch under ultrasonic agitation. Figure 25 shows the sequence of operational steps. Pinhole-free Si02 films can be produced by either of three techniques, thermal oxidation, CVD, and sputtering, respectively. When using a sputtering technique a minimum thickness of 100 nm is needed to form a dense oxide film. It should be noted that during the following diamond film deposition, which occurs in the presence of atomic hydrogen, He, a large amount of the Si02 coating is etched away by this very active species. Random growth of diamond could also be observed occasionally (Fig. 26), but to a much lesser extent than in the former case. No outgrowths are formed if the passivating oxide coating is dense and without pinholes, as is shown in Fig. 27. In the cross section of Fig. 28 the nearly structurless oxide mask can be distinguished very clearly laying side by side with the crystalline diamond coating, both of them growing on Si3N4 on Si.

778

Fig. 24: Fracture cross section of Si/Si3N4/D composite, patterned by the photoresist lift-off technique, showing areas of random growth of diamond (D) For a number of different applications such as mass flow meters, pressure transducers, thermistors, IR-windows and sensing devices, free standing membranes of diamond are required either as support and/or as sensing element. Figure 30 shows a cross section of SAD p-diamond on blanket diamond by way of a sputtered and patterned SiO2-mask. During the SAD of p-doped diamond the Si02-film is attacked; the formation of silicon carbide, Sic, and/or borosilicate glass, cannot be excluded. The oxide surface in this case is randomly covered with diamond nuclei and clusters of early growth shown in Fig. 31. However, the SiO2-oxide film as well as the randomly grown clusters of diamond can easily be removed by HF etch after coating and thus present no problem.

In all these cases the deposition of diamond and p-doped diamond is performed by thermal filament CVD (TF-CVD) or by pwave/electron cyclotron resonance-CVD (pW/ECR-CVD) on wa-

Silioon Wafer (100)

LPCVD Nitride Si,N, Thermal or spunered Oxide SiO, Positive Photoresist Panerniq Develop Photoresist Etch Oxide (BHF)

A) Diamond/p-Diamond B) Si3N4 C) Silicon

D) Si02 mask

Diamond Pretreatment

Strip Photoresist

SAD Diamond Coatings

-

--

Etch Oxide (BHF)

SCmn %N4

so2

Photoresist lhamond

rn plhamond

Fig. 25: Sequence of operations of diamond patterning with a Si02 mask fers of 4" diameter with a coating uniformity o f f 5% of the average film thickness. The substrate temperature was kept at all times at 800"C, the filament temperature at 2300°C f 100°C when using TF-CVD . The growth rate under these conditions is of the order of 1 p/h. These deposition conditions provided a constant and reproducible diamond quality of low impurity (originating from the filament material) as well as of graphite, sp2. The coatings are of basaltic structure (Fig. 30) with a strong (100) texture, and with an average crystal size of the order of 600 nm at 1 p coating thickness. The coatings are fairly smooth, with an average roughness,

Fig. 28: Fracture cross section of SAD diamond produced by the photoresist lift-off technique using a SiO2 mask on Si3N4 on Si wafer When using the pW/ECR-CVD method, diamond has been deposited at temperatures as low as 400OC, at an input power of up to 1500 W and at a pressure of 2.10-2 mbar. Mirror-like films of nano-crystalline diamond are obtained; the growth rate is very low, of the order of 0.1 pm/h. Sensor elements, obviously, need electrical contacts. To make such contacts to diamond coatings is not trivial. Good ohmic contacts to diamond films have been made with a Ti: 300 nm / Pt: 50 nm metal sandwich deposited by vacuum evaporation followed by a post heat treatment (66). During heating, interdiffusion occurs and atomic scale T i c bonds are formed between diamond and Ti/Pt. These contacts were patterned with a wet chemical etching process. The bonding to the TO5 holder is made with gold wire bonding. In the case of A1 wire bonding, an Au layer of 50 nm is deposited in addition to the Ti/Pt contacts to improve the adhesion of the A1 wires with the contact pads. In Fig. 33, an actual example of a sensor device, destined to be industrialized, in the geometrical form of a metallized meander structure of SAD p-diamond on Si3N4 on Si wafer, ready to be mounted in an anemometer, is presented for Siiioan Wafer (100)

LPCVDNitride SiN, Diamond Pretreatment

Diamond Coating

Spmered Oxide Si02 Positive Photoresist

Panerning Develop Photoresist Etch Oxide (BHF) Strip Photoresist

Fig. 26: Random growth of diamond on S i a masked Si

-

SAD pDiamond Coating

-

Etch Oxiie (BHF)

Sifimn

SioN4 Si02

Photoresist Diamond

m pDiamond

Fig. 29: Sequence of operations for SAD of p-doped diamond on blanket diamond with a Si02 mask illustration. This sensor device has been produced following the operational steps according to the scheme in Fig. 25, i.e. SAD using a Si02 mask. The ratio of the electrical resistances between doped and undoped diamond films is typically three orders of magnitudes or more. Fig. 27: Patterned diamond film on Si using a perfect Si02 mask, i.e. without pinholes Ra, of 0.1 pn (Fig. 32) as indicated by an AFM micrograph obtained by atomic force microscopy (74).

The temperature characteristics of the meander resistor, R vs T, was measured with a p-diamond thermistor sensor encapsulated on a TO5 standard package in the temperature range from 25°C to 600°C. in air, and up to 1000°C under Ar (65,66). A negative temperature characteristic has been measured on these thermistors up to 600°C in air with very good linearity up to 200°C. as shown in Fig. 34. In comparison with conventional thermistors the tem-

779

l.O

A) Si02 mask

\\

R vs T Thermistor characteristic

1

B) p-Diamond C ) Diamond D) Silicon

Fig. 3 0 Fracture cross section of SAD p-diamond on Si3N4 on Si, using a Si02 mask on blanket coated diamond

0.0 -

I

I

I

I

I

I

thus performing a thermal treatment at even higher temperature, does neither destroy the meander nor modifies the temperature vs resistance behaviour of the sensor element, nor leads to a hysteresis loop (75). A) p-Diamond B) Si02 mask C) Random nucleation Fig. 31: Top view of p-doped diamond with randomly nucleated diamond on the Si02 mask perature coefficientaD of p-doped diamond is very small, indeed. However, the useful ternuerature range is extended and allows excellent linearity down into the negative temperature range. The

Fig. 32: AFM - image of diamond film grown on Si

The current vs voltage characteristic. I/V.was measured with thermistor structures & described, i.e. with p-diamond on blanket diamond on SijN4-membrane and free standing p-diamond meanders on blanket diamond, respectively. Both types of thermistors, supported or not, can be heated repetitively f&n room temperature to 60O0C, i.e. red glow temperature, in air, without changing the (66). For illustration purposes, an integrated gas-flow transducer has been fabricated to demonstrate the feasibility (66). The device as depicted in Fig. 35 is based on two diamond thermistors and one heating resistor, either Pt or p-diamond. In the sketched device controlled heat is introduced to the gas stream by the Pt (or pdiamond) heating resistor in the middle part of the flow device, while the two diamond thermistor structures to the right and the left of it sense the temperatures before and after the heating stage. The differential measurement between the two thermistor elements is measured by using a Wheatstone bridge circuit configuration. The transducer operation is based on heat transfer. The upstream resistor indicates the temperature of the in-flowing gas, which, obviously, has to be kept at a given, constant 'temperature. In the direction of flow the gas is heated by a heating element, Pt or pdiamond, the heat pick-up being registered by the downstream resistor. From the temperature difference the quantity of gas flowing per unit time can be calculated, taking into account the heat capacity of the given gas or gas mixture. The conformance and reliability of the device has been checked with two Pt thermistors to ensure the correct functioning of the diamond based anemometer. Diamond-based type thermistors will find their industrial application whenever flows of gases or liquids have to be measured and controlled which are chemically aggresive and/or at high temperatures. doped diamond film

1

undopod dlornorbd lllm

I

.

. OUT

- -1

housing

Fig. 33: Metal contacted p-diamond meander structure for use as a thermistor element lower limit of temperature for which the resistance vs temperature characteristic is linear has not been determined as yet. Rising the temperature to 1OOO"C under Ar, i.e. to yellow hot temperature,

780

upstream raslrtor

heatlng raslrlor downstream resistor

Fig. 35: Anemometer design using two diamond thermistors

CONCLUSIONS

References

Not unlike the situation in the field of ceramic, high Tc superconductors, R+D in low pressure diamond synthesis has reached a state of disillusionment, characterized by the fact, that the feasibility of synthesis and of various applications have been shown, but that the technology and economics of production of items in many interesting industrial applications still has to be proven.

Celii, F.G., and Butler, J.E., 1992, Diamond Chemical Vapour Deposition, Naval Research Reviews, Vol. 44,No 3: 23-45

Though in certain cases such as diamond coated tools, the processes have been escalated to production or near production scale, it would be exagerated to t a k about a breakthrough, as was the case when, in 1969, SANDVIK AB came on the market with T i c coated cemented carbide inserts, developed by LSRH, a predecessor of CSEM, and, some two years later with Tic-A1203 and still another few years later with TiC-Al203-TiN coated inserts. Here the limiting factor is good adhesion behaviour of diamond coatings on cemented carbides and steel when produced in large industrial batches.

Hirose, Y., 1986, Synthesis of Diamond Thin Films by Thermal CVD Using Organic Compounds, Japan Journal of Applied Physics, Vol. 25: 519-521

Similarly but even less advanced is the use of CVD diamond films for the sake of electronic, optic and sensor applications where large scale production and post deposition fabrication treatment is at its best only at the beginning. Here the purity of the diamond and its structure - epitaxy is ultimately wanted - play the determining role. In order to get the industry heavily engaged in CVDlow pressure diamond synthesis as a production tool, a considerable amount of work on process escalation has to be done. Hence, sponsors and industry get impatient to some extent. In order to get further work sponsored, a breakthrough is needed, somewhere, in any field, for the production of an industrial pro,duct. This is what we hope for. Best candidates are: cutting tools, knives, scalpels, passive electronic components (such as heat sinks), sensor devices, and hopefully not too far in the future, fast, high power, low noise transistors and IC's (see e.g. 76).

Harris, D.C., 1992, Diamond: The Ultimate Durable Infrared Window Material, ibid, 3-16

Komanduri, R., Fehrenbacher, L.L., Hanssen, L.M., Morrish, A., Snail, K.A., Thorpe, T., Butler, J.E., Rath, B.A., 1990, Polycrystalline Diamond Films and Single Crystal Diamonds Grown by Combustion Synthesis, Annals of CIRP, Vol. 39/1: 585-588 Alers, P., Hanni, W., Hintermann, H.E., 1992, A Comparative Study of Laminar and Turbulent OxygenAcetylene Flames for Diamond Deposition, Diamond and Related Materials, Vol. 2: 393-396 Huong, Ph.V., 1991, Structural Studies of Diamond Films and Ultrahard Materials by Raman and Micro-Raman Spectroscopies, Diamond and Related Materials, Vol. 1: 3341 Komanduri, R. and Desai, J.D., 1984, Tool Materials for Machining, The Carbide and Tool Journal, Vol. 16 No 1: 3-1 1 Jennings, M., 1987, The Production and Uses of Industrial Diamond, Metals and Materials, September '87: 525-531 Okuzumi, F., Matsuda, J. and O'oka, K., 1990, Gaseous Phase Synthesis of Diamond and its Practical Application, Science and Technology of New Diamond, KTK Scientific Publishers, 149-153

General conclusions Diamond, sp3, can be produced by low pressure CVD by different methods. The quality of the coatings differ strongly from one method to the other. For industrial purposes the processeses need to be specified in order to get reliably reproducible properties. Though diamond coatings are presently used in different industrial applications such as cutting tools, heat sinks, sensor devices, scratch resistant optical coatings, machine elements for tribology, a true break-through of diamond coating applications in neither of these domains has been attained yet.

In view of industrial applications what is needed most is: (i) better adherence to functional substrates, mainly for mechanical applications, (ii) higher uniformity and homogeneity of the coatings over large areas, (iii) increased deposition rates, (iv) higher purity, (v) higher nucleation density, (vi) more randomly oriented structures, (vii) lower surface roughness, (viii) more efficient polishing methods, (ix) improved electrical contacting of the diamond layers, (x) epitaxial layers, (xi) three dimensional growth of large single crystals, (xii) whiskers and/or fibrous growth of diamond.

Kikuchi, N., Eto, H., Okamura, T. and Yoshimura, H., 1991, Diamond Coated Inserts: Characteristics and Performance, Applications of Diamond Films and Related Materials, Elsevier Science Publishers B.V., 61-68 Ito, T., 1991, Diamond Coated Cutting Tools Synthesized from CO, Applications of Diamond Films and Related Materials, Elsevier Science Publishers B.V., 77-83 Okuzumi, F., Matsuda, J. and Ooka, K., 1991, New Group of Tools with Thick Diamond Film Maae by Chemical Vapor Deposition, Superabrasive '91 Conference, Chicago, Illinois, 15/1-14 Nakamura, T., Fujimore, N., Nakai, T., Nakatani, S., 1991, Tool Applications of CVD Diamond, Superabrasive '91 Conference, Chicago, Illinois, 15/15-29 14) Yazu, S. and Nakai, T., 1991, Tool Application of Diamond and CBN, Application of Diamond Films and Related . Materials, Elsevier Science Publishers B.V., 37-41 15) Soderberg, S, Westergren, K., Reineck, I., Ekholm, P.E. and Shahani, H., 1991, Properties and Performance of Diamond Coated Ceramic Cutting Tools, Applications of Diamond Films and Related Materials, Elsevier Science Publishers B.V., 43-51. (16) Ravi, K.V., Plano, L.S. Peters, M. and Yokota, S., 1990, Controlled Microstructure Diamond Films - Synthesis and Selected Applications, Science and Technology of New Diamond, KTK Scientific Publishers, 29-38 (17) Kennametal Inc., Documentation on Diamond Tools

781

(18) Hosomi, S. and Yoshida, I., 1991, Diamond CVD Researches as Patent Applied, Application of Diamond Films and Related Materials, Elsevier Science Publishers B.V., 15-24

(37) Usuki, H., Narutaki, N., Yamane, Y., Karasuno, S., Ito, T., 1991, A Study on the Cutting Performance of Diamond Coated Tools, International Journal of Japan Society of Precision Engineering, Vol. 25 No 1: 35-36

(19) Norton Diamond Film, Cutting Tool Pamphlet (20) Man, D., Gonseth, D.R., 1993, A New Look at Carbide Tool Life, Wear, Vol. 165: 9-17 (21) Craig, P., 1992, Thin Film Diamond Derby, Cutting Tool Engineering, February '92: 23-31 (22) Mehlmann, A.K., Dirnfeld, S.F., Avigal, Y., 1992, Investigation of Low-Pressure Diamond Deposition on Cemented Carbides, Diamond and Related Materials, Vol. 1: 600-604

(38) 0-otake, N., Tokura, H., Yoshikawa, M., Yang, C.F, 1990, Deposition of Diamond Film on a Sintered Diamond Surface and its Application to a Cutting Tool, Science and Technology of New Diamond, KTK Scientific Publishers, 139-143 (39) Sen, P.K., 1992, CVDITE - A New Type of Cutting Tool Insert, Industrial Diamond Review, Vol. 5: 228-230 (40) De Beers Industrial Diamond Division, Documentation "CVDITE"

-

(23) Huang, T.H., Kuo, C.T., Chang, C.S., Kao, C.T., Wen, H.Y., 1992, Tribological Behaviours of the Diamond-Coated Cemented Carbide Tools with Various Cobalt Contents, Diamond and Related Materials, Vol. 1: 594-599

(41) Yoshikawa, M., 0-otake, N., 1988, Diamond Growth by Vapor Deposition and its Application to Cutting Tools, Bulletin of Japan Society of Precision Engineering, Vol. 22 NO 3, 171-176

(24) Huang, T.H. Kuo, C.T., Lin, T.S., 1993, Tribological Behaviour of Chemical Vapour Deposition Diamond Films on Various Cutting Tools, Surface and Coatings Technology, Vol. 56: 105-108

(42) Hay, R.A., Dean, C.D., 1991, Cutting Tool Performance of CVD Thick Film Diamond, Applications of Diamond Films and Related Materials, Elsevier Science Publishers B.V., 5360

(25) Yagi, M., 1990, Cutting Performance of Diamond Tool on A1-18 Mass% Si Alloy, Science and Technology of New Diamond, KTK Scientific Publishers, 399-404

(43) Murakawa, M, Takeuchi, S., 1991, Application of Brazed Diamond Film as Insert for Turning Aluminium Alloys, Surface and Coatings Technology, Vol. 47: 654-661

(26) Technology Briefs - Improved Adhesion strength of Diamond Films, 1993, American Ceramic Society Bulletin, March '93:114

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