RF Reactive Magnetron Sputtering Chamber

3.1.1.1. RF Power Subsystem

Figure 3.3 shows the RF power system, which includes the RF plasma generator and automatic matching network, respectively. The RF plasma generator is a Model GHW-12 (ENI. Inc.) and the type of automatic matching network is Model MW-10D (ENI. Inc.). The RF plasma generator is operating at the radio frequency of 13.56 MHz and producing 1250 W of maximum power into a 50 ohm load. The forward RF power and reflected RF power maintain a constant power output within ± 2% of the set point

over a dynamic power range of 0 to 1250 W by the DSP-based control module. A new 9-pin digital interface provides remote control, monitoring and diagnostic capability via the RS-233 serial link to a computer or host terminal. In addition, the automatic matching network is an automatic impedance matching network designed to interface with the ENI 13.56 MHz RF plasma generator to obtain plasma. The MW-10D includes two separate assemblies, such as a tuner unit and a controller unit. The tuner unit contains the matching network whose output impedance is varied by two motor-controller capacitors and a fixed inductor. The controller unit drives the tuner motors in response to signals on the remote interface or RF sensors in the tuner unit. In order to improve accuracy and tuning precision, the MW-10D uses a microprocessor-based control circuit. These two instruments can generate and maintain the plasma easier for sputtering processes.

3.1.1.2. The Magnetron Subsystem

The top and side view of the magnetron system is shown in Fig. 3.4. This magnetron system is constructed from a magnet assembly consisting of an iron plate (with a diameter of 8.3 cm) on which is mounted a series of magnets (1.1 × 0.6 × 2.6 cm) with the entire assembly mounted vertically under the target. It is therefore able to provide the magnetic field for confining the secondary electron motion to enhance the plasma density. The magnetic field strength was measured by the Lakeshore 421

Gaussmeter. This gaussmeter indicated a maximum perpendicular magnetic field of 350 G from the target surface along the center line and a maximum parallel magnetic field of 220 G from the substrate at 2.2 cm along the center line.

3.1.1.3. The Temperature Control Subsystem

The temperature was controlled by a temperature controller system (Model UP150, Yokogawa M&C Corporation) as shown in Fig. 3.5. This temperature controller was a heated substrate holder with lamp-type heaters in the back and a PID temperature controller. The quartz lamp heater is used in a vacuum environmental. The maximum temperature is 800℃. The lifetime of the heater decreases when the temperature rises over 800℃ or is operated in a non-vacuum environment. The controller panel can be used to adjust the rotation of substrate from 5 to 60 rpm.

3.1.1.4. Mass Flow Control Subsystem

Fig. 3.6 illustrates how mass flow rates of argon or mixture gases are controlled by

mass flow controllers (MFC). It consists of a thermal mass flow sensor, a rapidly acting valve, and an electronic control system, such as the Protec PC-540, featuring a multi-channel mass flow control by Sierra Instruments Inc. The purpose of the mass flow controller is to maintain a constant, operator-set flow. The maximum mass flow is 100sccm (standard cubic centimeters per minute). This system provides four channels for the simultaneous mixture of various gases.

3.1.1.5. Pumping Subsystem

The chamber is facilitated with a mechanical pump for obtaining low vacuum and a cryogenic pump for maintaining high vacuum as shown in Fig. 3.7(a) and Fig. 3.7(b), respectively. The rotary vane pump (mechanical pump) is an oil-sealed pump commercially available to pump gas with a pressure range of 760 torr – 10^-3 torr. A cryogenic pump captures molecules on a cooled surface by weak Van der Waals or

dispersion forces. It is used in a wide range of applications and in many forms. Liquid nitrogen or liquid helium “cold fingers” are used in the high vacuum chamber. Liquid cryogens or closed-cycle helium gas refrigerators are used to cool high- and ultra-high- vacuum cryogenic pumps [155]. The cryogenic pump model U-6H (ULVAC Technologies, Inc.) operated at a pressure range from 10^-3 torr to 10^-10 torr.

3.1.1.6. Pressure Control Subsystem and Gauge Meter Components

The MKS pressure controller, along with the ionization and low vacuum gauge controller, are the pressure control systems in our laboratory. The MKS Type 651C pressure controller (MKS Instruments, Inc.) is a self-tuning pressure controller for throttle valves as shown in Fig. 3.8. This system takes into account time constants, transfer functions of the valve and plumbing, valve gain, pump speed, and many other important parameters when determining the system characteristics. The default window display on the front panel exhibits the pressure readout and the valve position (% open).

The pressure readout can be displayed in units of Torr, mTorr, mbar, μbar, Pascal, or kPa. There are five reprogrammable set points provided, each one having the option of being set up for pressure or position control. Valve open, close, and stop functions are also provided on the front panel for use in system setup and diagnostics. The ionization and low vacuum gauge controller (Terranova Model 934) (see Fig. 3.9) provides pressure measurements across a broad range of vacuum environments. This system has the simultaneous advantageous of easy usability, an intuitive front panel and a large, bright, digital display. And because most of the needed features are built-in, this controller is comparatively low-priced. It also connects with the ion gauge and low-vacuum gauge. This controller automatically ranges throughout its operating range of 1 torr to 10^-11 torr.

There are three additional pressure gauges in our sputtering system, including a Baratron capacitance manometer, thermocouple gauge and ion gauge. Figure 3.10 exhibits the type 127 Baratron capacitance manometer (MKS Instruments, Inc.). A capacitance manometer is simply a diaphragm gauge in which the deflection of the

diaphragm is measured by observing the change in capacitance between it and a fixed counterelectrode [155]. This capacitance manometer is able to detect the range of pressure from 10^-3 torr to 10^-1 torr. The thermocouple gauge measures pressure-dependent heat flow, as shown in Fig. 3.11. It is generally used in a

low-vacuum environmental with an operating range of pressure from 10^-3 torr to 999 torr. The ion gauge, also called the Bayard-Alpert ion gauge, is shown in Fig. 3.12. It generally operates in the high and ultrahigh vacuum region, because extremely small particle density makes it almost impossible to operate. However, in the region below 7.5 × 10^-3 torr, pressure is measured by ionization gases. Hence, each ion gauge has its own lower pressure limit at which the ionized particle current is equal to a residual or background current. The ion gauge of our laboratory normally operated in the high vacuum region with a background limit from 10^-3 torr to 2 × 10^-10 torr.

3.1.1.7. Target and Gases

In general, the sputtering method can obtain ZnO thin film by using a metal target of zinc (Zn) and ceramic target of zinc oxide (ZnO). In this study, we introduced the Zn metal target to grow ZnO thin film, because the Zn metal target has several advantages over the ZnO ceramic target. The advantages are: (1) the metal target is much purer than the ceramic target; (2) it has the best cooling efficiency because of its higher thermal conductivity; and (3) the metal target is cheaper than the compound ZnO ceramic target.

We determined that the purity of the Zn target is 99.999% (5N) with a 4 inch in.

diameter and 0.25 inch in thickness (Sinoxp Materials Company). The Zn target has 3 mm thickness of Zn bond, and a 3 mm thickness of copper (Cu) plate on the bottom side.

A typical reactive sputtering system is composed of a metallic target with working

(discharge)/reactive gas species. Essentially, the working (discharge) and reactive gas are argon (Ar) and oxygen (O2), respectively. The atoms of reactive gas will combine with the sputtered atoms from the target to form the compound film of ZnO on the substrate. In this study, the purity of Ar and O2 are both 99.999%. In order to obtain the ZnO films, these two gases are mixed prior to entering the sputtering chamber with a predetermined ratio in volume flow rate.