PROTECTIVE CVD COATINGS FOR THE TOOL INDUSTRY : REQUIREMENTS FOR PROCESS CONTROL AND EQUIPMENT

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PROTECTIVE CVD COATINGS FOR THE TOOL INDUSTRY : REQUIREMENTS FOR PROCESS

CONTROL AND EQUIPMENT

E. Mohn, R. Bonetti, H. Wiprächtiger

To cite this version:

E. Mohn, R. Bonetti, H. Wiprächtiger. PROTECTIVE CVD COATINGS FOR THE TOOL INDUS-

TRY : REQUIREMENTS FOR PROCESS CONTROL AND EQUIPMENT. Journal de Physique

Colloques, 1989, 50 (C5), pp.C5-811-C5-819. �10.1051/jphyscol:1989598�. �jpa-00229631�

JOURNAL DE PHYSIQUE'

Colloque C5, suppl6ment au n05, Tome 50, mai 1989

PROTECTIVE CVD COATINGS FOR THE TOOL INDUSTRY : REQUIREMENTS FOR PROCESS CONTROL AND EQUIPMENT

E. MOHN, R. BONETTI and H. W I P ~ C H T I G E R

Berna AG Olten, Industriestrasse 36, CH-4600 Olten, Switzerland

R 6 s u m 6

Les plus importants critGres de construction d'un Gquipement de dgposi- tion par CVD sont descrits. La fiabilitg et reproductibilits des dGp6ts sont avant tout garanties par un choix judicieux des composants, et par une construction qui facilite l'utilisation et les travaux de maintenan- ce. D'autre part, la souplesse d'utilisation qui permet une production de masse ainsi que des travaux de d6veloppement sophistiquGs, est assu- rGe par un contr6le informatique qui donne S l'utilisateur tous les degrGs de libert6 pour concevoir des cycles de traitement en respectant les conditions ngcessaires pour la protection des hommes, de l'gquipe- ment e t d e la c h a r g e .

Abstract

The most important design principles of a modern CVD cqating equipment are described. viLbility and reproducibility of the deposited layers are mainly given by a careful choice of high quality components, as well as by a design which allows for easy service and maintenance done within regular time intervals. A high versatility which allows to perform routine mass production as well as sophisticated development work is assured by a programmable computer control which gives the user the required freedom to design his process cycles without restrictions except for those required by safety aspects for people, equipment and load.

1

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INTRODUCTION

About twenty years ago, the first CVD coated cutting tools were introduced for increased efficiency in machining operations. By now, over 60 % of all carbide inserts sold by the tool industry are of the coated type. Whereas for a long time single layer coatings such as TiN, Tic and Ti(C N ) have peen dominating,

X Y

and the usefulness of coated inserts has been limited to turning operations, new materials such as A1203 and combined coatings (TiC/TiN, TiC/A1203/TiN, etc.) were added later, and more and more coated tools useful for interrupted cutting and milling became available as well.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989598

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Today, the four compounds Tic, TiN, Ti(C N ) and A1203 may be considered as

X Y

standard materials for CVD deposits. Due to their intrinsic prqperties with respect to resistance against different types of wear and oxidisation, which are to some extent complementary, suitable combinations may be designed to respond to almost any requirements met in practical machining operations. More exotic compounds such as HFN, ZrN etc. are occasionally offered for special purposes.

The major progress in the performance of coated cutting tools has however been characterized by careful optimization of the entire system including the base carbide material, cutting geometry, machining parameters and the coating layer. Some examples for the latter include

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Multilayer coatings (Fig. la) consisting of between 10 and 20 individual layers not more than 1 um thick, often alternating between A1203 and some other suitable intermediary layer. It is claimed that frequent Interruption of the growth process leads to better grain size control and superior per- formance, provided that adhesive problems between individual layers can be avoided /l/.

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Using organic compounds as a donor for N and C instead of molecular gases such as N2 and CH4, the deposition temperature can be lowered by 200

-

^300°

C. This results in more favourable microstructures of Ti(C N ) coatings X Y

(Fig. lb), with well controlled and constant x:y ratios and a lower tendency of decarburization of the substrate, leading to superior performance in interrupted cuttlng operations /2/ and /3/.

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Using compounds of group Va or VIa elements as catalysers and dopants, A1203 can be deposited with higher growth rates, finegrained microstructure and less edge build-up ("dog-boning") /4/.

Fig. 1 a) Fig. 1 b)

SEM cross section of a CVD multilayer SEM cross section of a typical MT-Ti coating, consisting of 12 individual (C ,N ) layer with characteristic layers with A1203, and showing fine- negdlg-type grain structure (scale grained structure due to repeated 5500:l).

growth interruptions (scale 5500:l).

Industrial production of such refined and sophisticated coatings calls for a corresponding, advanced equipment. A high degree of process control, homo- geneous coating characteristics within large reactor volumes and run to run reproducibility is required to meet the demands of a user equipped with modern, eventually unmanned machining centres. As important as a high mean .value of the cutting amount is a small standard deviation of the character- istic lifetime, in order to avoid machine stand-stills because of premature tool failures.

The purpose of this paper is to demonstrate that CVD deposition systems can be designed to meet these requirements and to describe the essential features which p-rovide a solution to the problems mentioned above.

2

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SPECIFICATIONS

The most important specifications, to which a modern CVD-coating equipment should respond, may be summarized as follows:

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Reliable, reproducible coating results within the daily routine production.

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Handling by unskilled operators, with no need for permanent supervision.

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High availability, easy maintenance and servicing.

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High productivity resulting in low per piece costs.

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Sophisticated prograding flexibility which allows the user to implement his proprietary process know-how and to develop new types of coating.

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Modular design in order to tailor the system to specific user needs with respect to coating capacity, degree of automatization and materials to be deposited.

With systems of this type having become available, it seems to be a tendency that even big users are moving away from in-house developped coating units in favour of commercially available systems, the resources required for such an R&D capacity becoming prohibitive.

3

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HARDWARE CONCEPT

To a large extent, the suitable design of the equipment hardware and careful selection of high-quality components determines whether a consistent coating quality is obtained reproducibly run after run. Extensive model calculations /5/ have been done to demonstrate the influence of reactor and load geometry, including the gas in- and outlet system, and of the process parameters on the local distribution of growth rates. Though the usefulness of these studies is somewhat limited by the necessity to assure laminar flow patterns and by in- complete knowledge of thermo-dynamical and kinetic data for the involved reactions, a number of general design principles and hints are provided.

CS-814 JOURNAL DE PHYSIQUE

Definitely, operating at sub-atmospheric pressure helps to equalize growth rates over large reactor volumes due to higher values of diffusion coeffi- cients, which are inversely proportional to pressure. A lower residence time at equivalent gas flows allows for faster heat-up of the entering gases. To- gether with a suitable gas distribution system, coating thickness can be held within

+/-

15

. . .

^{20 %} or even lower over reactor volumes as large as 500 mm diameter by 1000 mm length. Also, the availability and use of powerful, re- liable liquid ring vacuum pumps takes care of the by-products elimination problem. By proper monitoring and regulating the pH-value of the neutralizing liquid, emission values for toxic gases and solid particles leaving exhaust ventilation may be well kept below the strictest emission standards enforced by law in all industrialized countries.

Precise metering of gases and gaseous educts is exclusively done by mass flow controllers. These are available for all type of gases and flow ranges, and are well compatible with a computerized process controller. Since the measu- ring principle using a small capillary in parallel to the main flow is not entirely free from problems due to clogging or diameter changes by residues or deposits, adequate means for rinsing and calibrating should be available.

Also, standard rotameters may be provided in series connection to allow vlsual control of the correct operation of MFC's. Liquid precursors are preferably dosed by mechanical pumps while still in the liquid phase, evaporation into the carrier gas occuring only after the metering device. All the necessary educts are brought together in a collector which is placed as closely as pos- sible to the reactor inlet, using heated gas lines in order to avoid any con- densation. The temperature difference between the (water cooled) reactor flange and the first useful level for coating substrates within the reactor amounts to approximately 800° C over a distance of some 30

...

^{40 cm.}

Additional preheating of the reactants by passing them trough a labyrinth lo- cated within this space has found to be very beneficial for optimum coating results already on the bottom-most level. Further, for coating of densly packed, small components such as carbide inserts, it is advantageous to have a central gas outlet on each load level (tray system). In order to avoid imaging of flow patterns on the substrates, the central gas distribution tube can be rotated around its longitudinal axis at a few rpm's.

Another important point is the availability of a clean, inert or reducing gas atmosphere for heating up, cooling down and intermediary cleaning phases bet- ween deposition of two different materials. This is provided by use of a switchable bypass gas line, which circumvents all reactive component gene- rators and the collector, and enters the main gas line immediately beneath the bottom reactor flange.

Finally, the entire gas preparation, distribution and evacuation system is subdivided into modular units, which can individually be accessed for clean- ing, servicing and leak testing, in part even while the coating system is ope- rating. This allows not only for easy and efficient maintenance, but in many cases to save a run and the valuable load in case that a problem somewhere in

the system should occur.

4

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^CONTROL AND SOFTWARE CONCEPT

Consistent coating results in mass production are most readily obtained by complete automatization of the process cycle. Arbitrary operator influences such as entering and controlling set values on analog devices, or more or less conscious changes of the process parameters, are completely eliminated.

It should be recognised that even the most skilful and scrupulous operator can hardly avoid a certain degree of non-reproducibility when for instance he has to take a manual system from one deposition regime to another, characterized by different process parameters such as temperature, pressure, and/or gas types and flows.

On the other hand, the designer must be careful not to pre-empt what the user finally wishes to do with his equipment. Even if process parameters for each individual coating phase could be programmed by the user, preconceived ideas about how to manage the transfer between different coating materials, or about measures to be taken in abnormal situations ("emergency programs"), lead to undesirable constraints and lack of transparency. The only solution which really offers the desirable flexibility is a freely programmable software structure which allows the user to create process "recipes" by adding step after step in any sequence and with any choice of process parameters.

Free programming essentially means that every regulating element of the system has to be set either by digital (valve positions, activation of precursor generators, etc.) or analog output signals (mass flows, furnace zone tempera- tures, pressure, etc.). A consequent realization of this principle implies that for each programme step some 30 to 40 parameters would have to be entered by the user. Even for simple coating cycles, the programming effort would be very time consuming and awkward. A way out of the dilemma of reconciling un- limited flexibility and economic recipes creation has been found by definition of two separate programming levels. On the top (operator accessed) level, only those parameters are entered which are immediately related to the coating pro- cess itself (p, T, mass flows and a few digital orders), as well as an addi- tional parameter called system status.

The informations contained within a specific status are mainly the following:

actual configuration of the equipment, valve positions, operating sequence to establish a status, allowable transitions to other status, alarm bandwidths and emergency routines. The status definitions are accessible to the user as well on a lower, keyword protected engineering level. Thus, he may change the standard values provided by the manufacturer, or later adapt them to eventual hardware changes or additions.

C5-816 JOURNAL DE PHYSIQUE

The number of emergency r m k i n e s has been reduced to what is absolutely neces- sary to protect people, equipment and load from irreversible damage. For the same purpose, hardware interlocks are established where necessary. In conse- quence, *not every conceivable malfunctioning is taken care of and automati- cally h h d l e d by the system. Experience shows that in many cases a manual intervention can save a run which would otherwise be lost. Also, system set up and running in, short test runs and maintenance are most easily handled by manual operation. Therefore, decentralized hardware controllers for all feed- back-loops and manual actuator switching elements are implemented in addition to the PC-controlled software. Simple turning of a couple of switches allows to change from automatic to partially or totally manual operation and vice versa. This feature also guarantees that the entire unit can still be operated in case of a PC break down. A data collection routine provides informations about run conditions for both automatic and manual mode. This data can later be retrieved and displayed by a graphic printer or on the PC screen.

The process control software, recipe handling and extensive data collection are implemented on an industrial version of a powerful PC system. In order to illustrate the software philosophy, two examples of a processing sequence are given in Figs 2 and 3. Figure 2 shows the initial four steps of a coating recipe named 01HT3.

The first step is the heating up phase, being performed in the equipment status BYPAP, i.e. using the by-pass line to fill the reactor with a N2/H2 mixture still at atmospheric pressure. Step duration 0 simply means that the next step will only be initiated when the desired set temperature of 800° C has been obtained. Within the next step (status BYPLP), the pressure is low- ered to 200 mbar, the temperature changed to pure H2, still using the bypass line. Again, step 2 will be terminated when the new set temperature has been arrived at. Step 3 is a carburizing phase during 30 min at a pressure of 100 mbar, characterized by a linear increase of temperature from 980° C to 1000°

C. Finally, in step 4, Tic is deposited during 60 min at still increasing temperature. Since we now use a reactive educt, the system status must be changed to activate the corresponding generator and gas lines (DIRLP).

Figure 3 is another sequence illustrating the transit from a Tic to a TiN deposition. Note from steps 6 and 8 that not only temperatures, but also gas flows ( N 2 in step 6) and pressure (in step 8) can be linearly varied over a desired time period. It is important to know that, as soon as the step dura- tion is set above 0 , upon initiation of a new step its parameters will imme- diately become active. In order to avoid unnecessary alarm messages or even emergency actions, which are only due to the time delay until the hardware regulators have fitted the effective to the new set values, each alarm may be delayed by the desired amount of time, this delay also being part of the informations registered in the status tables.

RECIPE NAME: O I H T 3 STATUS: BYPAP STEP NUMBER: I S t e p d u r a t i o n 0 m i n

b e g i n e n d

I

b e g i n e n d

I

G e n e r a t o r 0 0 d e g C 0 0 a l / m ~ n

T l C i 4 0.00 0.00 m l / m ~ n

P r e s s u r e 0 0 m b a r

I

^{CH3CN H2s 0} 0 m l / m ~ n

Z o n e 1 8 0 0 8 0 0 d e g C

Z o n e 2 8 0 0 8 0 0 d e g C

Zone 3 8 0 0 8 0 0 d e g C

Zone 4 8 0 0 8 0 0 d e g C

H 2 Gen. 0.0 0.0 I / m i n

HCI Gen. 0.00 0.00 I / m i n

RGS: 1 1 / 0 c o o l i n g N 2 : 0 1 / 0

CH3CN-heat i n g : 0 1 / 0 c o o l i n g H2: 0 1 / 0

H 2 2 . 5 2 . 5 I / m i n

N 2 5 . 0 5 . 0 I / m i n

CH4 0.00 0.00 I / e : n

HC I 0.00 0.00 i / m i n

C 0 2 0.00 0 . 0 0 I / m i n

RECIPE W E : O l H T 3 STATUS: BYPLP STEP NUFIBER: 2

S t e p d u r a t ~ o n 0 man b e g l n e n d

Z o n e 1 9 8 0 9 8 0 d e g C

Z o n e 2 9 8 0 9 8 0 d e g C

Zone 3 9 8 0 9 8 0 d e g C

Zone 4 9 8 0 9 8 0 d e g C

G e n e r a t o r 0 0 d e g C

P r e s s u r e 2 0 0 2 0 0 m b a r

S t e p d u r a t i o n 3 0 m i n

Fig. 2 Segment of a coating cycle recipe with initial heating start-up and transition to Tic deposition. The pressure value of 0 mbar in step 1 is not relevant, because status "BYPAP" defines the system to operate at atmospheric pressure.

b e g l n e n d

H 2 8 . 0 8 . 0 I/man

N 2 0.0 0.0 i / m l n

CH4 0.00 0.00 I / m l n

HC i 0.00 0.00 I / m l n

C 0 2 0.00 0 . 0 0 I / m i n

H2S 0 0 m l / m ~ n

T : C1 4 0.00 0.00 m i / m l n

CH3CN 0 0 m l / m t n

H 2 Gen. 0.0 0.0 i / m ~ n

HCI Gen. 0.00 0.00 I / m ~ n

b e g i n e n d Z o n e 1 9 8 0 1 0 0 0 d e g C Z o n e 2 9 8 0 1 0 0 0 d e g C

Z o n e 3 9 8 0 1 O O O d e g C

Z o n e 4 9 8 0 1 0 0 0 d e g C

G e n e r a t o r 0 0 d e g C

P r e s s u r e 1 0 0 i00 m b a r

RECIPE NAME: 0 1 H T 3 STATUS: D I R L P STEP NUMBER: 4

S t e p d u r a t i o n 6 0 m i n

RGS: 1 1 / 0 c o o l l n g N Z : 0 1 / 0

C H 3 C N - h e a t l n g : 0 1 / 0 c o o l i n g H 2 : 0 1 / 0

RECIPE NAME: 0 1 H T 3 STATUS: BYPLP STEP NUMBER: 3

b e g i n e n d

H 2 13.0 1 3 . 0 I / m i n

N 2 0 . 0 0.0 i / m i n

CH4 1 . 5 0 1 . 5 0 I / m i n

HC I 0 . 0 0 0 . 0 0 I / m i n

C 0 2 0 . 0 0 0.00 I / m i n

H2S 0 0 n i / m i n

T i c 1 4 0 . 0 0 0.00 m l / m i n

CH3CN 0 0 m l / m i n

H 2 Gen. 0 . 0 0 . 0 i / m i n

HCI Gen. 0 . 0 0 0.00 I / m i n

b e g i n e n d Z o n e 1 1 0 0 0 1 0 2 0 d e g C Z o n e 2 1 0 0 0 1 0 2 0 d e g C Z o n e 3 1 0 0 0 1 0 2 0 d e g C Z o n e 4 1 0 0 0 1 0 2 0 d e g C

G e n e r a t o r 0 0 d e g C

P r e s s u r e 8 0 8 0 m b a r

RGS: 1 1 / 0 c o o l i n g N 2 : 0 I / O

CH3CN-heat i n g : 0 1 / 0 c o o l i n g H 2 : 0 1 / 0

b e g i n e n d

H 2 1 2 . 0 1 2 . 0 i / m i n

N 2 0 . 0 0 . 0 I / m i n

CH4 1 . 1 0 1 . 1 0 i / m i n

HC I 0 . 0 0 0.00 I / m i n

C02 0 . 0 0 0.00 I / m i n

H2S 0 0 m l / m i n

T i c 1 4 0 . 8 0 0 . 8 0 m i / m i n

CH3CN 0 0 m l / m i n

H 2 Gen. 0.0 0 . 0 I / m i n

HCI Gen. 0 . 0 0 0.00 l / m i n

RGS: 1 1 / 0 c o o l i n g N Z : 0 l / O

CH3CN-heat i n g : 0 1 / 0 c o o l i n g H 2 : 0 1 / 0 ( C t r i .H) => n e l p

C5-818 JOURNAL DE PHYSIQUE

R E C I P E NAME: 0 9 H T 3 STATUS : D1 R L P STEP NUMBER: 5

/

S t e p d u r a t i o n 5 0 n i n

R E C I P E NAME: 05'HTZ STATUS : D I H L P STEP NUMBER: 6

1

S t e p d u r a t i o n 1 5 r r ~ i n

I

b e g i n e n d

1

b e g i n e n d

1

b e g i n e n d

Z o n e 1 l 0 1 0

Z o n e 2 1 C) 1 C)

Z o n e 3 1 0 1 0

1 ^{Z o n e} 4 1 0 1 0

G e n e r a t o r 0 0 d e g C

1 Oi>

P r e s s u r e lO(3 m b a r

I

b e g i n e n d

H 2 1 t . C 1 6 . 0 l i m i n

N Z _{CH4 1} 0.0 _{.QO 0.90} 0

.

^{0 I /m} l / m ! n ^{i n}

HCJ, O.oO 0.00 I / r i ~ r n

L0 L 0.00 0.00 I / m i n

H 2 S 0 0 m l / r n j n

T i C I . 1 0.80 0 . 8 0 m l / m i n

CH3i:I.I 0 0 m l / r n ~ n

H 2 G e n . 0.0 0 . 0 I / m i n

H C I G e n . 0.00 0.00 I / m i n

1 1 / 0 l cuol i n a N 2 : 0 1/'0

3 C N - h e a t i n u : V 1 / 0 c o o l i n a H 2 : O 1 / 0

1

! R G S : 1 1 / 0 c o a l i n o N Z : 0 1 / 0

l C H Z C N - h e a t i n g : U 1 / 0 c o o l ins H 2 : O 1 / 0

Z o n e j 1,320 1 0 ' 0 d e r C

1i:)36 d e i C:

Z o n e L 1 0 2 0

Z o n e 3 1 0 2 0 1 0 2 0 d e a C:

1i:)20 d e g C

Z o n e 4 1 0 2 0

G e n e r a t o r 0 0 d e g C

1 0 0

P r e ssur c 1 0 0 m b a r

I I

R E C I P E NAME: O Y H T 3 STATUS : D1 R L P STEP NUMBER: 7

H 2 1 6 . 0 1 6 . 0 I / m j n

N 2 l .<S c.

.

^{(3 I /m i n}

CH4 0 . 7 0 O . 9 0 I / m j r t

HC I 0.00 0.00 I / m r n

C 0 2 0.00 0.00 I/'mi71

H Z S 0 0 i n l / r n ! n

T i C1 4 0 . 8 0 0 . 8 0 m l / r n ~ n

CH3CN 0 0 m l /m i n

H 2 G e n . 0 ,0 0.0 I / m i n

H C I G e n . 0.M) 0.00 I l r n i n

1

S t e p d u r a t i o n 1 6 5 m i n

1

I

b e g i n e n d

1

b e g i n e n d

I

I

RGS : 1 1 / 0 c u o l i n u N 2 : 0 1 / 0

C H 3 C N - h e a t i n a : O 1 / 0 c o o l i n 6 H 2 : 0 l / 0 Z o n e j 1 0 2 0 1 0 0 0 d e g C

Z o n e L 1 0 2 0 1 0 O 0 d e g C Z o n e 3 1 0 2 0 1 0 0 0 d e a C Z o n e 4 l 0 2 0 1 0 0 0 d e G C

G e n e r a t o r 0 0 d e g C

P r e s s u r e l 00 1 0 0 r i ~ b a r

H 2 1 6 . 0 1 6 . 0 I./rnin

N 2 8 .O 3.0 I / m i n

CH4 0 . 4 0 0 . 7 0 I / r n i n

H C I 0.00 0.00 I / m i n

0.00 0.00 I / m i n

0 0 m l / m i n

T i C1 4 0.70 0 . 7 0 m l / m j n

CH3CN 0 0 r n l / r n ~ n

H 2 G e n . 0.0 0.0 I / m / n

H C I G e n . 0.90 0.00 I / m ~ n

Z o n e 1 Z o n e 2 Z o n e 3 Z o n e 4

P P

R E C I P E HATME: 0 9 H T 3 STATUS : D1 R L P STEP NUMBER: 8

S t e p d u r a t ~ o n 15 m i n

V W m ~ / m ~ n

0 .0 0.0 I / m ~ ! n 0.00 0.00 I / r n l n

RGS : 1 1 / 0 c o o l i n q N 2 : 0 1 / 0

C H 3 C N - h e a t i n a : 0 1 / 0 c o o l i n Q H 2 : 0 1 / 0 b e g i n e n d

Fig. 3 Segment of a coating cycle showing transition from Tic to TiN deposi- tion.

b e g i n e n d

Obviously, this solution does not work for temperature changes due to the con- siderabl'e thermal inertia of the system. If a substantial temperature diffe- rence between two subsequent steps is involved, the user has to program a transition step

-

usually with a linear temperature ramp

-

allowing sufficient time for the desired change.

At first sight, this procedure seems to be somewhat clumsy, but experience has shown that it is in fact very beneficial, since it invites the user to consi- der carefully and define precisely what should happen with the other parame- ters during the transition phase. Consistent and well controlled coating re- sults are the award for this intellectual effort.

5

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CONCLUSIONS

It has been shown that rational design principles and application of advanced hard- and software engineering allow to realize CVD coating equipment, which is well suited for mass production and sophisticated development work. Whereas it is mainly the hardware elements which decide upon the prospect to obtain consistent highquality coating results, a high degree of versatility and re- producibility is obtained by the free programmable control system described in this paper. Figure 4 shows an example of a single system with one reactor base for a 325 mm diameter reactor.

Fig. 4 Automatic CVD unit with one reactor base for a 325 mm reactor, gas and power control cabinet, and PC based control system.

By careful value analysis of the entire system as well as every individual component, it has become possible to offer these systems at costs which are largely compensated for by the added value provided to tool products by a suitable coating.

REFERENCES

/l/ Schintlmeister, W., Kanz, J. and Wollgram, W., Refractory and Hard Metals 3 (1983), 41

-

43.

/2/ Kubel, E., VD1 Berichte 670 (1988), 625

-

^636.

/3/ Bonetti-Lang, M - , ~onetti,~., Hintermann, H.E. and Lohmann, D., Proc.

Eighth Int. Conf. on CVD (19811, 540.

/4/ Lindstrom, J. and Smlth, U - , EP 0 045 291 (1984).

/5/ Wahl, G., Proc. of EURO-CVD Four (1983), 19

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35.