JUSTIFICATION OF THE OPERATING MODE OF HIGH-VACUUM SPUTTER-ION TITANIUM SORPTION PUMPS

V. P. Ponomarev

Introduction

Titanium ion-sorption pumps, whose operation is based on the sorption of gases by a sputtered titanium film, have certain advantages over diffusion vapor-jet pumps \[1\], which has led to their widespread use for producing high vacuum \((P = 10^{-6}—10^{-10}\ \text{torr})\) in various physical apparatus and industrial installations.

Sputtering of titanium in pumps of low and medium throughput \((S = 10—50\ \text{l/sec})\) is often carried out by thermal evaporation with additional ionization of residual gases or by electron bombardment. Both in these and in other designs of titanium pumps \[3—5\], complete data are not given on the justification and selection of the operating mode of the pumps, which ultimately reduces the effectiveness of their use.

The article makes an attempt to justify the operating mode of pumps with a throughput of \(S = 30—60\ \text{l/sec}\) with titanium sputtering by electron bombardment.

The data obtained are the result of an experimental study of pump designs made in a diode configuration.

Fig. 1. Experimental design of the electrode system of a titanium pump: 1—titanium collector (anode); 2—tungsten cathode

Selection of the Shape of the Pump Electrodes

The properties of pump electrode systems were investigated on the system schematically shown in Fig. 1. It consists of a collector (anode) made of VT-1D grade titanium and a tungsten cathode.

The investigation of the indicated electrode system made it possible to draw the following conclusions:

1. For sputtering titanium sufficient for pump operation, the specific power released on the titanium collector must be within the range of \(50—100\ \text{W/cm}^2\) for a collector thickness of \(1—5\ \text{mm}\) (Fig. 2).

1. Increasing the service life of the pump requires a larger amount of titanium placed in the pump. This leads to an increase in the power dissipated at the collector and, moreover, with a greater initial thickness of the collector, as it is sputtered the cathode–collector distance increases. Therefore, in order to maintain the specified titanium sputtering regime—that is, the electron current in the cathode–collector circuit—it is necessary either to increase the accelerating voltage or to decrease the cathode–collector distance. These drawbacks are eliminated by dividing the titanium reserve in the pump into 2–4 parts, which are sputtered in succession. This makes it possible to obtain a pump service life, in terms of the titanium reserve, of 500–1000 hours at low supplied power; at the same time, adjustment of the accelerating voltage or of the cathode–collector distance during operation is not required.

Fig. 2. Dependence of the specific power released at the collector on its thickness at different rates of titanium evaporation.

1. With a planar electrode system, the titanium collector is sputtered nonuniformly. It is sputtered most intensively at the center because of the higher density of the electrode current incident upon it. Elimination of this drawback is possible by using a semicylindrical design of the electrode system (Fig. 3), which under our conditions was adopted as the principal version of the electrode system of titanium pumps [5] in the diode configuration.

Fig. 3. Semicylindrical design of the electrode system of a titanium pump.

Pump operating mode

The operating parameters of titanium pumps depend to a considerable extent on their power-supply circuit. The cathode supply circuit does not exert a special influence on the operation of a titanium pump. This cannot be said

of the second element of the circuit, which creates the electric field accelerating the electrons. The nature of the voltage that creates the field, and its magnitude, have a determining influence on the pumping speed of the pump, which is the principal operating parameter of the pump.

The electrode system of a pump with the titanium reserve divided into several parts makes it possible to use both alternating and constant accelerating voltage. In addition, the relation between the accelerating voltage and the geometry of the electrode system determines the magnitude of the electron current in the collector circuit.

For a semi-cylindrical electrode design, the dependence between the quantities can be expressed by the formula [6]

\[ I_e = 2.33 \cdot 10^{-6}\,\frac{Q_{\mathrm{eff}}}{r^2 \beta^2}\cdot U^{3/2}\ [a], \]

where
\(I_e\) is the current in the collector circuit, in A;
\(r\) is the distance between the cathodes and the collector, in cm;
\(Q_{\mathrm{eff}}\) is the collector area, in cm;
\(U\) is the accelerating voltage, in volts;
\(\beta\) is a coefficient allowing for the shape of the electrodes.

The optimal choice and ratio among these quantities determine the operating mode of the pump.

In the graph: \(S\), l/sec; \(U\), V. Legend: \(o\)—constant voltage, \(I=130\) mA; \(\times\)—alternating, \(I=60\) mA; \(\square\)—alternating, \(I=60\) mA; \(Q_{\mathrm{eff}}=\mathrm{const}\).

Fig. 4. Graph of the pumping speed of the titanium pump as a function of the type and magnitude of the accelerating voltage

The dependence of the maximum pumping speed of the pump on the magnitude and type of the applied accelerating voltage is shown in Fig. 4.

From an analysis of the curves obtained it may be concluded that a rapid increase in the pumping speed of the pump is observed at a voltage \(U>800\)--900 volts. This voltage value is the minimum permissible one for pump operation, at which the electrons acquire kinetic energy sufficient for sputtering the titanium collector. At voltages of 900–1200 V, the conditions for titanium sputtering are most favorable, since, along with the electron flux, radiant energy from the heated cathode also participates in the sputtering of the titanium collector, because the distance \(r\) is small.

Increasing the accelerating voltage above 1200 V is inadvisable, since the small gain in the pumping speed of the pump is achieved by increasing the accelerating voltage.

It is clear from the graph what advantages are achieved when the pump is supplied with a constant voltage. With alternating voltage, the electrons have sufficient energy to sputter titanium in the central part of the voltage half-wave, which also determines the lower pumping speed under alternating voltage.

An important operating parameter of a titanium pump is stability in maintaining the specified titanium sputtering mode. The determining influence on the titanium sputtering mode is exerted by the magnitude of the current \(I_e\), and, consequently, the problem reduces to maintaining the specified current in the collector circuit at a certain value of the accelerating voltage. As this current limiter, maintaining it at the specified value, use is made of the space charge formed by the electrons near the cathode.

Fig. 5. Graph of the emission characteristic of the cathode of a titanium pump

The formation of a space charge at a specified value of the current \(I_e\) is accomplished by setting, during assembly of the electrode system, the required cathode–collector distance \(r\), calculated by formula 1. This position is confirmed by the graph of the emission characteristic of the cathode of the titanium pump (Fig. 5).

There are two characteristic regions on the curve of the graph. Region I characterizes the amount of increase in current calculated by formula 1. At point 1, a space charge is formed, and with an increase in cathode filament heating the magnitude of the electron current \(I_e\) changes almost not at all—region II.

Point 2 has been adopted as the operating point of the pump; at this point the cathode filament voltage, in comparison with point 1, is increased by 15–20%.

Conclusion

As a result of selecting the electrode systems of pumps with titanium sputtering by electron bombardment and studying the operating mode of the pumps, the following conclusions can be drawn:

1. The semicylindrical design of the electrode system most fully satisfies the operating conditions in the pump.

1. Dividing the titanium reserve in the pump into 2–4 parts makes it possible to double the service life of the pump, while reducing the power consumed for titanium sputtering by a factor of 2–3 compared with pumps in which the titanium is concentrated in a single collector.

2. Selecting the operating point of the pump in the space-charge mode ensures its long-term stable operation without additional adjustments.

3. Titanium ion-sorption pumps with a pumping speed of \(S = 30\text{–}60\ \text{l/sec}\) are recommended to be supplied with a constant accelerating voltage of \(\dot{U} = 1100\text{–}1200\ \text{V}\); in this case the requirements for the stability and ripple of the rectified voltage are the most general.

REFERENCES

1. Contemporary Vacuum Technology. Il., 1963.
2. M. I. Men’shikov, PTE, 4, 3, 1959.
3. A. G. Mishkin, “Elektronika,” 11, 62, 1958.
4. I. A. Grishaev et al., PTE, 3, 144, 1960.
5. V. P. Ponomarev, PTE, 6, 143, 1963.
6. A. F. Vlasov, Electron and Ion Devices. GEI, 1960.