Magna TM 7 magnetron sputtering system in creation of thin-film microwave hybrid ICs
"Магна ТМ 7". Рассмотрены ее устройство и принцип работы. На установке получены тонкие резистивные пленки на прямоугольных поликоровых пластинах ВК-100-1 магнетронным распылением дисковых мишеней диаметром 100 мм из материалов РС3710 и РС5406. Четырехзондовым методом установлено, что значения удельного поверхностного сопротивления полученных пленочных резисторов могут достигать 100 Ом/кв.
The following characteristics of a film of resistive material are important for creation of thin-film resistors:
• resistivity of the film, its reproducibility and stability in time;
• specific power dissipation of the film;
• temperature coefficient of resistance (TCR);
• performance specifications (noise spectrum and level, etc.).
An important task is creation of the domestic reliable special process equipment, which could provide the required quality of film resistors and allow to operate in automatic mode with control of all parameters at all stages of the deposition process. This article provides basic information about the design and technological capabilities of the new Magna TM 7 system designed for vacuum deposition of multilayer conductive and resistive thin films by magnetron sputtering.
Fig.1 shows an appearance of the system (a) and its schematic diagram (b). The system consists of the process unit and the power and control rack. Process unit (Fig.2) includes a working chamber 4, which is fixed on the frame 1, assembled of aluminum profiles. Chamber with an internal diameter of 380 mm and a height of 230 mm is made of stainless steel and has cooling channels for hot elements. Four flanges for installation of the various autonomous process devices are located on the generatrix of the chamber. In this version of the system, a heating lamp (not shown), ion source (IS) 3, and two magnetrons 2 and 5 are used for the magnetron sputtering deposition of the thin resistive films.
The system operates as follows. Seventeen wafers with a size of 48 Ч 60 Ч 2 mm are fixed on the substrate holder (Fig.3a), which is attached to the upper flange of the vacuum chamber and provided with a motor reducer that ensures smooth rotation of the wafers with a frequency of up to 20 rpm. A lifting mechanism is used for easy loading and unloading of samples (Fig.3b). At first, electric drive lifts the substrate holder over the vacuum chamber, at a distance of 1 200 mm from the floor level, and then, manually, using a rotary mechanism, it is led away from the vacuum chamber on distance convenient for loading or unloading of the wafers.
Before loading of the wafers, the working chamber is filled with inert gas, such as an atomic nitrogen N2, through puffing valve with electromagnetic control. A throttling valve with manual adjustment is used for regulation of flow.
After loading of the wafers, the working chamber is vacuumized using the vacuum system, which consists of the backing scroll pump located in the vicinity of the process unit. The vacuum system also includes high vacuum valve with the possibility of throttling the pumped volume, the turbopump and pneumatic controlled vacuum valves with a bellows seal located near the working chamber.
Different gauges are used for controlling the residual pressure, operating pressure, pressure at the outlet nozzle of the turbopump. Dry vacuum system provides a residual pressure in the working chamber better than 5 ∙ 10–4 Pa, 30 min after the operation of the turbopump at full capacity. To reduce the process time, the heater is switched on at a vacuum of 8 ∙ 10–3 Pa, 10 min after the start of vacuumizing. The lamp heater (Fig.4a), which is composed of four halogen lamps with a capacity of 0.5 kW each, is used for heating the wafers. Power is supplied to the heaters through a vacuum electrical input from the heating control unit. The thermal resistance sensor registers the pre-heating of the wafers to 100 °C, and in the process of deposition and subsequent annealing the heating reaches 300 °C. The objective of preheating is the removal of residual water vapor from the wafers. After that, the wafers are subjected to ion bombardment for removal of various impurities to improve the adhesion strength of thin films with the surface of the samples. For pre-cleaning of substrates is used having a rectangular aperture of the ion source (Fig.4b) with a closed electron drift with supply of positive high voltage potential up to 3 kV to the anode from the power supply. The ion source (Fig.4b) with closed electron drift, having a rectangular aperture, with applying a positive high voltage potential up to 3 kV to the anode from the power supply is used for the pre-cleaning of substrates. Central and peripheral pole tips of IS, as well as the case have a grounded potential. The magnetic system is assembled from separate cylinders with size of 20 Ч 40 mm, made of neodymium-iron-boron alloy. All elements are water cooled to prevent overheating and failure of the IS.
One of the two magnetrons is switched on after the pre- (finish) treatment for a short period of time, to clean the target from contamination. All elements inside the chamber are protected from the flow of sputtered material during target training using the shutter attached to the rotational drive, which is attached to the lower flange of the working chamber and opens the devices according to the sequence of processing (Fig.5a). The planar magnetron sputtering systems (Fig.5b) with circular targets with a diameter of 100 mm and a thickness of 6 mm are used for generating the flow of the deposited substance. The targets are clamped to a water-cooled base, made of copper, through the indium gasket. The magnetic system is assembled from magnets of rectangular cross section, made of neodymium-iron-boron alloy. The magnetic circuit made of carbon steel is used for closing the magnetic field. Magnets with a magnetic core are placed in an aluminium alloy case, which is washed with water. Using the tesla meter, it was found that the induction of magnetic field on the surface of the target is 0.1 T. Magnetrons are equipped with protective screens for stable burning of the discharge and to protect the internal chamber space from the dust.
A DC unit with output power up to 3 kW and a maximum current of 7 A is used to power the magnetrons. The unit has a power output connector that is galvanically isolated from the case. A potential of negative polarity of up to 650 V is applied to the target for deposition of thin films, and the ignition of discharge occurs at voltages of up to 1 200 V. A switching power device is used to switch power between the magnetrons.
The valve opens the prepared target of one of magnetrons and the lamp heater, and the target material is deposited on the wafers with their simultaneous heating. The latter increases the adhesion strength of the target material with the surface atoms of the samples and allows to adjust the structure of the deposited thin films and, accordingly, their surface resistivity. A witness wafer is used for the control of temperature and specific surface resistance of the samples during the deposition. Deposition is performed until the specific surface resistance on the witness wafer will not exceed 100 Ω.
The use of two magnetrons in the Magna TM 7 allows to combine deposited materials and to deposit multilayer thin films during one cycle of vacuuming. Then an annealing of the wafers for 30 min at temperatures of up to 300 °C is conducted for crystallization of structure of the formed resistive coatings. After completion of the process the chamber is filled with inert gas, and the wafers are unloaded.
The facility is equipped with compressed air supply system. Pneumatic panel has an air preparation unit or dehumidifier, pressure gauge for control of input pressure and electronic distribution device for supplying compressed air to the required pneumatic controlled actuators.
The system of cooling water supply to the heating elements includes a special header that provides multiple channels of supply. Hydro-panel has a sensor to monitor the input pressure and an electronic device for controlling the flow of water and its output temperature on each channel.
The gas system is equipped with three channels for the supply of gases with flow regulators on each of them. The argon is fed through the first channel, the second channel is designed to feed oxygen into the ion source, and the third channel is aimed to feed an inert gas into the working chamber at the beginning or at the end of the process.
The power supply and control rack comprises ion source power supply, magnetron power supply, heating control unit, as well as various switching elements. The operator controls the process via the display, a computer mouse and keyboard in manual or semiautomatic modes. An industrial computer is used for processing data from sensors and generating control signals. In the case of emergencies and power outages, the operation of control system will be supported by uninterruptible power supply. The turbopump controller is also installed here.
The Magna TM 7 was used for deposition of thin film resistors using circular targets of resistive alloys RS3710 and RS5406 on 60 Ч 48 mm rectangular wafers of VK-100-1 with a thickness from 0.5 to 2 mm. The process was carried out at operating pressure of argon of 0.1–1.0 Pa, the sputtering power for each target from 1.5 to 3 kW and heating temperature up to 300 °C. By four-probe measurements it was found that specific surface resistance of the obtained film resistors can achieve 100 Ω/square with dispersion not more than ± 3.5%.
This considered system allows to deposit thin film resistors in the production of MW HIC using magnetron sputtering. The location of the wafers on the substrate holder in the vertical plane ensures the deposition of high-quality thin films with good adhesion strength between substrate and adhesive layer. Automated control system of Magna TM 7 monitors all process parameters and supports their stability according to a settings providing a good reproducibility of the properties of the deposited coating. ■