Combinatorial Thin Film Deposition

COMBINATORIAL DEPOSITION OF THIN FILM MEMBRANES FOR HYDROGEN SEPARATION AND THEIR CHARACTERIZATION

This chapter consists of five sections. The magnetron sputtering system which was developed for the combinatorial study is described in the first section. A brief information on the thin film deposition of membranes via magnetron sputtering is also included in this section. This is followed by a section on material characterization techniques used in the current study. The third section deals with four-probe resistivity measurement used as a screening method to identify candidate compositions for the separation membranes. This is followed by a section which describes the methods used in fabricating selected membranes in the form of foils.

The last section comprises a custom-made setup for permeability measurement.

4.1 Combinatorial Thin Film Deposition

A magnetron sputtering system was designed and constructed for the combinatorial thin film deposition. The system had a vacuum chamber of 458 mm diameter and 400 mm height, Figure 4.1. The system comprises three sputter guns in a triangular geometry with the center to center distance of 200 mm. The guns with dedicated shutters are installed in the chamber with high vacuum O-ring based feedthroughs which would allow vertical adjustments. The gun head could also be adjusted by tilting.

The chamber was connected to a turbomolecular pump backed by a rotary pump which can provide a base pressure down to 5x10^-8 Torr. The chamber had quartz

40

thickness sensors, with a sensitivity of ±0.1 Å, mounted next to each gun. All connections types in the chamber are conflate (CF), except the sputter guns and the main door.

(a)

(b)

Figure 4.1 The view of the magnetron sputtering system and (b) a view from a typical simultaneous deposition.

The chamber had a gas inlet which allows an individual or simultaneous feeding of argon, oxygen, and nitrogen gases for ordinary or reactive sputtering. Each gas is controlled by an individual thermal mass flow controller which allows up to 50 sccm

41

flow rate. The system is connected to a PC which allows the control of the sputtering parameters.

Two of the sputter guns were connected with an RF (300 watts) power supply, while the other one was connected to a DC (600 V, 2A) source. The system as constructed would allow the simultaneous deposition from the three targets, though if needed sequential deposition in the form of multilayers is also possible.

The substrate holder with dedicated shutter was typically 70 mm above the guns and could accommodate substrates up to 6-inch diameter. The system allows the rotation of substrate holder up to 8 rpm. It also has a facility for substrate heating with the use of quartz heating elements which could allow temperatures as high as 400 ºC. In the combinatorial study, the substrate holder was in the form of multiple sample holder accommodating substrates of 18 mm in diameter. A schematic representation of the multiple substrate holder is given Figure 4.2. The holder allows mounting of 21 substrates in a triangular fashion mimicking the geometry of the guns underneath.

Figure 4.2 Multi-sample holder designed for combinatorial thin film deposition.

4.1.1 Calibration for combinatorial thin film deposition

The combinatorial thin film deposition relies on compositional variation which would be obtained as a result of a substrate having locations that vary with respect to

42

targets underneath. The sputtering system was carefully adjusted so that all thin films would have similar thicknesses while yielding different compositions. Therefore, a deposition rate of 1:3 was aimed, referring to the sample at the center and the sample at the corner of the triangle, see Figure 4.2. In this way, the simultaneous deposition of three targets, each at the same deposition rate, would result in a uniform thickness distribution throughout all 21 samples.

Figure 4.3 The traces of the Kapton tape removed from the glass substrate.

A series of calibration experiments were carried out for each target material used in the study to ensure the 1:3 ratio. The calibration experiments were performed on 6-inch plain glass substrates. Initially, 6 mm Kapton tape was adhered onto the glass substrate in a triangular geometry to imitate the actual holder geometry. The glass substrate was then loaded to the sample holder in the vacuum chamber and each sputtering target was positioned underneath at the corners of the substrate triangle.

Typically, a 30 min of deposition was performed using only one target at a rate of 2Å/sec. The tapes on the glass substrate were then removed, Figure 4.3. The thickness of the films was measured along the traces of the Kapton tapes using a profilometer (Veeco, Dektak 6M). The film deposition and thickness measurements were repeated in cycles. Each sputter gun was aligned both vertically and angularly, until the thickness ratio of 1:3 was achieved.

43 4.1.2 Thin film deposition of ternary alloys

In a typical thin film deposition of a ternary alloy, 150 µm thick glass substrates with 18 mm diameter were placed in the multiple sample holder and then loaded to the vacuum chamber. The chamber was then evacuated down to 5x10^-8 Torr and then was filled with ultra-high purity argon to 5mTorr.

Pre-sputtering was carried out for 10 minutes for surface cleaning of the target while the shutter was kept closed. Following the pre-sputtering, the gun shutters, as well as the main shutter, were then opened and the simultaneous deposition was carried out at a constant rate of 2 Å/s for each target for 250 min. Typically, each deposition yielded ~3 µm thickness for all samples.