Optimization of evaporation and sputtering devices through substrate heating parameter

Vacuum/volume 41/numbers 7-9/pages 2187 to 2189/1990

0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc

Printed in Great Britain

Optimization of evaporation and sputtering devices through substrate heating parameter A Z h u n d a , Vacuum Metallurgy Division, Special Design Office of Vacuum Coatings, Riga, USSR

In vacuum deposition processes, the substrate heating rate depends mainly on the type and design of an evaporation device and on the coating method. The present technique of measuring specific transfer heat makes it possible to find ways to minimize the heating rate or to raise it up to a certain level. The technique is based on direct mass measurements of the condensate applied onto the receiving surface of a sensor and on temperature gain measurements of the sensor during a fixed time interval. The present technique is applicable to any type of evaporation and sputtering devices.

1. Introduction The substrate heating rate is one of the most important vacuum coating process parameters affecting the coating equipment efficiency and coating quality. Because of highly complex nature of the processes involved and uncertain emissivity factor values of emitting and receiving surfaces, analytical methods of heat flux calculations are often prone to inadmissibly high errors. The same coating thickness may be obtained for different times by varying the condensation rate. Thermophysical parameters of the coating process will be different as well. From the efficiency point of view, most suitable are those process conditions which provide maximum transfer of evaporated material from an evaporation device to a substrate with the latter's minimum heating. In other words, those process conditions are preferable which ensure a minimum ratio of heat flux Q (which has reached the substrate and has heated it from temperature T 1 to temperature T2) to condensate mass m per unit time dT. The ratio Q/m = Hn is the specific heat of heat/mass transfer during the vacuum deposition process or, in si/npler terms, the specific transfer heat. Known values of T~ and T2 allow to determine the total heat flux onto the substrate, using the following formula: Cp, (T) d r

Q =~ I

where mn-substrate mass (g), Mn-molecular mass of substrate material (g m o l - ' ) , Cpn-heat capacity of substrate material (cal m o l - ' g - 1). Mass of the condensate applied is determined as the difference of the substrate weight before (ml) and after (m2) condensation, i.e. m ~m2--ml,

From ref 1 it is known that Cp = a + b T + c T -2.

Substituting the ratios thus derived into the transfer heat formula and making the subsequent integration, we obtain: nn =

mn {a(T 2 _ b 2 [ T t - T2"~] Mn(m2_ml ) TI) + ~(T2 - 72) - C t T-'-T-~.T~ ) ~.

The latter expression makes it evident that, to determine the specific transfer heat when the values of m n, M n, a, b and c are known, only three parameters are required, namely: pre-condensation temperature T~, substrate temperature after condensate application T2, mass of condensate deposited, m, as a difference between m e and ml.

2. Experimental method The most valid values are those obtained by direct measurements of parameters T~, T2 and m, performed under actual coating process conditions directly in the commercial vacuum coating equipment. Here, the most important prerequisite of measurements is to provide conditions for obtaining the highest accuracy. The objective is met by using a special purpose device designed for specific transfer heat determination. This device (Figure 1) comprises a metal body accommodating a heat-sensitive part with proper heat insulation. This heat-sensitive part consists of two coaxially mounted sensors made of the same material, preferably with high thermal conductivity, e.g. copper. The outer 40 mm diameter sensor is intended to be used for temperature measurements; it is provided with a drilled hole for fixed attachment of a thermocouple. The thermocouple junction is secured in the hole with a metered quantity of silver solder. The device is supplied with a cover plate having residual gas evacuation outlets to prevent spurious heat fluxes from affecting the measurements. The inner 16 mm diameter sensor is used for mass measurements and is made readily removable. Its receiving surface 2187

A Zhunda: Optimization of evaporation and sputtering devices 12

9

6

8

18

17 15 14

The temperature gain of inner sensor '5' positioned in the same heat/mass flux where the outer sensor (characterized by generic values AHn, qk and z) is located, will be equal to:

A T s - AHnqkfT -Cpml

m

Figure 1. Specific transfer heat measuring device. (1) body; (2) drilled hole for ceramic rod (3, 4) temperature measuring sensor; (5) mass measuring sensor; (6) thermocouple; (7) cover plate cap screw; (8) holder; (9) residual gas evacuation hole; (10) silver solder; (11) hole for thermocouple; (12) cover plate suppressing influence of spurious heat fluxes; (13) ceramic tube; (14) annular bore; (15) flanged edge; (16) condensate; (17) spring-loaded ceramic rod used for fixing sensor 5 in annular bore 14; (18) clamp; (19) annular bore. whereon condensate is deposited is made coplanar with the receiving surface of the outer sensor. To enable indifferent orientation of the device within the vacuum chamber, there is a spring-loaded ceramic rod fixing the position of the inner sensor. A specific transfer heat measurement comprises the following steps: the device with the mass of its inner surface registered by the analytical balance is positioned in the vapor flow space area to be investigated, a coating cycle is performed, with the coating temperature and time recorded, upon evacuation of the vacuum chamber, the inner sensor is removed and weighed, the specific transfer heat is calculated using the above formula. The superior measurement accuracy as compared, for instance, with probing techniques is due to high precision of condensate mass measurements, since there is no need to measure the mass together with the thermocouple or to detach it from the sensor to be weighed. The design of the present device is unique in that separate measurements of temperature (using the outer sensor) and mass (using the inner sensor) yield valid results provided the condition of equality between the ratios of condensation areas to masses of the sensors holds true, i.e. F

M~

f

ml

where F and M, are receiving surface area and mass of the outer sensor, respectively;f and rn~ are receiving surface area and mass of the inner sensor, respectively. To make sure that it is really so, let us consider the following. The temperature gain of outer sensor '4' may be described by the following expression: AT4 = AHnqkFZMl , Cpn

where AHn-specific transfer heat, qk -condensation rate, z -condensation time. 2188

As follows from the two last expressions, the equality AT4 = ATs holds true only if F/f= M1/ml. In a practical embodiment, this ratio is readily implemented by calculating the receiving areas of the sensors and by fitting the mass of the inner sensor at the expense of dimensions of an annular bore. Furthermore, the size of the gap b between the outer and inner sensors also plays an important role in the device design. When this gap is too small, condensates might merge, thus resulting in uncertainty of the boundary between condensates upon removal of the inner sensor. On the other hand, too large a gap results in a relatively high mass of condensate getting onto cylindrical end surfaces of the sensors. In any case, information about the true mass of condensate applied onto the receiving surface of the inner sensor will be distorted. The best results are achieved when the gap b is of the same order of magnitude as the applied condensate thickness A. 3. Results and discussions The present device was used to investigate thermophysical parameters of a number of heat evaporation devices and plasma ionization sputtering sources for evaporation (sputtering) of a number of metals. Table 1 shows some data related to magnetron sputtering. For these experiments, assemblies incorporating three or four devices in a single unit were used. When interaxial distance between adjacent sensors was 60mm, a three-device assembly covered a 140ram 2 area. A planar magnetron sputtering source incorporating a 136 mm diameter target and a balanced magnetic system with a powerful central magnet (type 13) was used in the experiments. Measurements were performed at argon pressure of 0.4 Pa within the power range of 1. . . . ,5 kW. The distance between the sensor surfaces and the target was 100 mm. Table 1 contains heat rate data; those taken from the literature are expressed in eV. atom -~ and coverted into k J . g - 1 , since the latter unit of measurement is more suitable for practical estimation of a heat/mass transfer process. The measurements have shown that the magnetron sputtering devices incorporating a balanced magnet system yield the lowest specific substrate heating rate directly in the central sputtering area ('Measurements' column in Table 1). The righthand column in the same part of Table 1 lists the specific transfer heat values for the distance of 60 mm from the target center. Higher experimental values as compared with theoretical ones confirm the conclusion that the heating share contributed by the bombardment by electrons and neutralized ions might amount to 10-60% of the total beat/mass transfer. Table 2 presents specific transfer heat values for aluminium when using different evaporation (sputtering) techniques. As can be seen from these data, the highest substrate heat rate is produced by the electric arc evaporation technique; as distinct to other techniques investigated, this heat rate in the central area is higher than in the periphery.

A Zhunda: Optimization of evaporation and sputtering devices

Table 1. Specific transfer heat for materials sputtered in planar magnetrOn sputtering device

Metal

aluminium copper titanium molybdenum stainless steel (XI8H10T)

Theoretical values

Experimental data

eV atom- ~

kJ g - ~

13a 12a 22b 26a .

46.3 18.1 44.1 26.0 .

.

literature eV atom- 1

kJ g - i

kJ g - l

measurements

11a 12a 20b 42a

39.2 18.1 40.1 42.1

56-68 21-29 70 - 73 45 -46

.

41-44

Data taken from ref 2. b Data taken from ref 4. a

I~K)

Table 2. Specific heat transfer for evaporation (sputtering) of alu-

minium Evaporation (sputtering) technique

120

Specific heat transfer, kJ g - 1 central 120 mm area distance Notes

I10 I00 v

electron beam

central

distance

resistivity electron beam magnetron

13 14 56

15 70 68a

magnetron

200

150a

electric arc

98

56

electric arc

560

210

unbalanced magnetic

~<

90

7O

balanced magnetic system unbalanced magnetic system sensor isolated from anode anode potential on sensor

a Sensor located at a 60 mm distance from the central substrate/magnetron sputtering source axis.

The electron beam evaporation data show a possibility to completely suppress the heat transfer contribution made by charged particle bombardment of the substrate ( 1 4 k J g - t ) . Still, the design version under study fails to provide sufficiently intensive removal of reflected electrons from the substrate, as indicated by a five-fold increase of the specific transfer heat value at a distance of only 120 m m from the central axis of the crucible (70 kJ g - l). An example is given below to illustrate how to use an assembly of specific transfer heat measuring devices for design optimization of a 348 m m diameter round cathode planar magnetron sputtering device with the aim of decreasing the substrate heating rate. Curve 1 (Figure 2) illustrates the specific transfer heat distribution along the radius of the magnetron sputtering device target. A higher specific transfer heat value reaching 128 kJ g - i indicates the occurrence of plasma bombardment against the substrate. When extra anodes are added, the specific transfer heat (curve 2) goes down to the level which is characteristic for the unbalanced magnetic system (53 . . . . . 66 kJ g-~).

6O 5O

60

120

180

rnm Figure 2. Specific transfer heat distribution along radius of magnetron

sputtering device. (1) non-optimized design; (2) design version with additional anodes.

4. Conclusion

The present method for measuring specific transfer heat and the devices developed for this purpose allow to obtain valid data for specific heating rate parameters of a substrate in a coating process. Numerical values characterize the integral heat flux per unit mass of the material condensed on a sensor and include the heat transfer by all energy types present in the process of evaporation (sputtering) and condensation, namely: condensation heat, kinetic energy of atoms, radiation from the evaporation surface (plasma), and energy of neutral particles.

References

1Properties of Elements, Part 1: Physical Properties. Metallurgiya, Moscow (1976). 2j A Thornton and J L Lamb, Th/n Solid Films, 119, 87 (1984). 3 B Window and N Sawides, J Vac Sci Technol, 196 (1986). 4j A Thornton, Thin Solid Films, 54, 23 (1978).

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