Dear Readers! This is your 4pvd Newsletter. We hope you enjoy the information it contains. If you wish to unsubscribe, please send a blank Email to unsubscribe@4pvd.de June 1 st, 2006, Issue No. 24 Principles of Vacuum Gauges 4pvd, Dr.-Ing. Stefan Esser The last newsletter explained, that for vacuum applications pump is not equal to pump, and for some vacuum pumps gas is not equal to gas. This is even more important if we start to measure the pressure in a vacuum chamber. The lower you get in pressure, the more difficult it becomes to get a good measure. Mechanical and Capacitive Gauges In theory pressure is force divided by the area it is applied on. At pressures higher than 1 mbar this force can be used to bent a diaphragm and move a pointer (Fig. 1 left). In the same range piezoelectric crystals can directly convert the force into an electric signal. At a pressure of 10-5 mbar the pressure on a diaphragm is only 1mN/m² or 1nN/mm² (1 nano- Newton / mm²!). Fig. 1: Mechanical vacuum gauge (left) and capacitive vacuum gauge (right) In spite of these unbelievable small forces there are very reliable vacuum meters available which detect the displacement of a very fine diaphragm down to pressure of 10-4 mbar. These small forces of course cannot move a pointer, but the displacement of the diaphragm causes a change of the electric field to an opposite electrode. This is recognized as a change of capacity of the sensor and that s why the sensors are called capacitive gauges. To make these systems work at the said low pressures, the whole gauge must be packaged and thermally conditioned. Venting to atmospheric pressure will cause a permanent deformation of the diaphragm and should be avoided. Therefore the low pressure capacitive gauges need a valve for protection and a second sensor, which opens the valve.
The mechanical systems can measure between atmospheric pressure (and above) and 10-1 mbar. Capacitive gauges can be used from atmospheric pressure down to 10-4 mbar but a single sensor usually can cover max. four decades. Both the mechanical and the capacitive gauges measure the real pressure, independent from the gas composition. The systems listed below measure side effects of the pressure and may deviate depending on the gas composition. Pirani Gauges You will find them everywhere. Pirani gauges measure pressure from atmospheric down to 10-3 mbar. A filament is heated moderately by an electric current. Depending on the gas pressure the filament transmits heat to the gas and its temperature changes. With the temperature change the electrical resistance of the filament changes and this allows a read out of the pressure. Fig. 2: Principle of Pirani Gauge and deviation of the display depending of the gases In general Pirani gauges are inexpensive and reliable and usually there is no maintenance necessary. As the signal depends on the thermal conductivity of the gas atmosphere there may be some significant deviation in the pressure range higher than 1mbar if gases other than air are measured (Fig. 2 right). In the range of evaporation processes the effect of thermal conduction is so low, that the accuracy of the measurement is usually not enough. Ionization Gauges For measurement at high vacuum, systems have been developed which maintain a constant glow discharge inside and measure the ion current between two electrodes. This ion current is depending on the pressure but also on the gas composition (The influence of the gas composition sometimes can be used for a basic leak detection, as ionization gauges show a deviation if e. g. helium is in the vacuum atmosphere.). There are basically two ways to maintain the constant glow discharge. Bayard Alpert or hot cathode systems use a filament, heated up to at least 1000 C by electric current and emitting large numbers of electrons (fig. 3 right). Penning or cold cathode systems use cold electrodes which are at potential difference of more than 1000V and are supported by a strong magnetic field (fig. 3 left).
Fig. 3: Principle design of cold (left) and hot (right) cathode ionization gauge Some hot cathode gauges can measure within a range of 5 decades. With the design of electrodes and materials the range can be shifted lower or higher but not enlarged. The hot filament usually does not like too much oxygen or strong vibrations. To increase the lifetime of the filament it should be heated only at pressures within the operation range, which requires a second (e.g. a Pirani) sensor for control. But still than most problems will be related to the filament. Cold cathode systems usually are more reliable, but also should not be started above 10-2 mbar. In modern sensors there is the high voltage power supply, the Pirani system for switch on and off and the electronics integrated in the sensor head. The advantage of this combination is, that the user does not need to care about all these issues and in the transition range between Pirani and Penning the electronics matches the signals of the two sensors. If you run penning heads for long time at higher pressures, the electrodes might get dirty due to material deposited from the glow discharge. As said the measurement of lower pressures is more difficult. The sensors need to be protected against influence from the PVD process or the environment. Avoid excessive heat in the cabinet but also radiation from the process. Magnetic field from the cathodes and their wiring influences especially the ionization gauges. Back streaming oil in the rough vacuum pipes may damage the Pirani gauges. On the other hand take care, that the sensors are close enough to the chamber or the measuring point. This includes that the flange dimensions are sufficient. For fine vacuum and process pressure KF10 usually is OK. For high vacuum better choose short KF40 flanges. Some sensors require a special orientation of the head, if not choose orientation, so no dust can fall into the sensor. As none of the above systems covers the whole pressure range necessary for a PVD process, usually combinations of two or more sensors are used. Very common is the combination of Pirani / Penning full range gauge together with a capacitive gauge for the evaporation process. Fig. 4 roughly shows what pressure ranges are covered by the different principles.
Fig. 4: Pressure ranges for various sensor types Sphere Fixtures for very small substrates If you have to coat small substrates with 3-fold rotation, maybe due to the requirements of film thickness distribution or adhesion, and if 2-fold rotation is not applicable, you might be in trouble. Your customer says: small parts, small price. But the substrate isn t any more small, if you look at it together with the fixture and the bearing and the necessary distance to the next level of substrates. In general the relative space occupied by fixtures goes up significantly when the size of the substrate goes down (fig. 5). Fig. 5: Relative utilization of coating zone depending of the substrate size The Sphere fixture design may help you to get out of this trouble. Sphere fixtures have the bearings and drive mechanism not in the primary coating zone, but hidden in the centre of the planetary tower. Especially for small substrates you can fill a lot more substrates in the coating zone compared to conventional fixture design.
Lets have a look at a small bolt or end mill with diameter 4,0mm and total length of 40mm. A conventional fixture plate with 130mm diameter may carry 30 pieces of it. If you build a stack out of these conventional plates, you have to add 10mm for the bearing and 10m distance to the total length ending up in a total height of 60mm at least per level of 30 substrates. Fig. 6: Sphere plates for substrates 4,0mm L=40mm The Sphere fixture for this kind of substrate also carries 30 pieces per level. But due to the tilt of the substrates which is 30 in this case the total height per level can be reduced to 25mm. This means more than double the batch capacity with the same distance substrate to substrate. But there are more advantages of the sphere design for this application: The tilt of the substrate rotation axle leads to higher film thickness on the tip The mechanism for the third axle is integrated in the plate and is always adjusted properly The tower can be loaded and unloaded w/o disassembling All dust particles from kickers, bearings etc. are trapped inside of the tower The deposition of coating material is pretty low in the fixture but high on the substrates Fairs and Conferences before the Summer: 20. 24. June METAV, Düsseldorf Coming up with July Newsletter: Standard Boxer Sizes on Stock now Principles of Pressure Regulation News, Fairs and Conference, Events, etc. Send a comment to Stefan.Esser@4pvd.de Please feel free to send this newsletter to a friend