Theory and Background
4.2 Sample Preparation
4.2.1 Substrate Cleaning
In our experiments, we use corning glass (number: 7059) as substrates. The width is 10 mm square and thickness is 0.3 mm. Before depositing films onto the substrates, we should clean the glass to avoid the unanticipated impurities on the surface. The cleaning processes are as follows. The substrates are cleaned in turn in trichloroethylene (TCE), acetone (ACE), and alcohol solvent for five minutes in an ultrasonic cleaner, and then the substrates are blown-dry with N2 gas. Once finish the cleaning, a mask will be used to pattern the shape of films. In this series of experiments, we used two kinds of masks, metal mask and photolithography.
4.2 Sample Preparation
4.2.1.1 Metal Mask
The width and the length of the films are 0.4 mm and 5.7 mm, respectively. The film thickness are 195˚A ± 5˚A or 147˚A ± 3˚A. Figure 4.1 shows the processes of the deposition using metal mask. All of the cartons are at front view.
Step A: Preparing a clean glass.
Step B: The metal mask is plastered on the glass using a thin coat of Apiezon
’N’ vacuum grease.
Step C: Depositing the wanted material and thickness onto the mask and glass. The deposited thickness should be thinner than the mask to avoid the connecting between the films on glass and on the mask.
Step D: Remove the metal mask carefully. The patterned films are deposited on the substrate.
4.2.1.2 Photolithography
Because of the difficulty of manufacture of metal mask, the narrowest width which one can achieve is about 0.1 mm by using metal mask. When one needs narrower films, the photolithography technique can help us achieve it. The limit of the width of photolithography technique depends on wavelength of light. Typically, the narrowest width is about several µm. Figure 4.2 shows the processes of the photolithography technique. There are five steps to achieve it.
Step A: Coating a photoresist, a liquid polymeric material, onto the substrates.
The coating process is performed by spinning the substrates at speeds several thousand rpm. Photoresist is deposited onto the substrate surface during the dynamic movement to ensure coating over the entire substrate surface. The coating thickness depends on the photoresist and spinning speeds.
Step B: Once the substrate has been coated with photoresist, putting the photo mask on the substrate and exposing the substrate on an exposure light.
By shining light through the photo mask and onto the substrate, individual areas of the photoresist are selectively exposed to light. This exposure causes a chemical change in the photo resist.
Step C: Once exposed, the substrate is then immersed in a developer solution.
Developer solution are typically aqueous and will dissolve away areas of the
pho-4.2 Sample Preparation
Figure 4.1: Processes of deposition by using metal mask.
4.2 Sample Preparation
toresist that were exposed to light. Therefore, after successful development, the photoresist is patterned with the wanted shape.
Step D: Depositing the wanted material and thickness onto the mask and glass. The deposited thickness should be thinner than the mask to avoid the connecting between the films on glass and on the mask.
Step E: Remove the photoresist by acetone. The patterned films are deposited on the substrate.
4.2.2 Sputtering
All of our thin films are fabricated by sputtering deposition. Our source target is the Cu93Ge4Au3 (atomic rate). Before sputtering, we pump the chamber vacuum to high vacuum. Once the vacuum reaches the order of 10−6 mbar, we inject 7 sccm argon gas into the chamber and the pressure increases to order of 1 × 10−3 mbar. Because our target is a good conductor, we chose a DC voltage source.
The sputtering power ranges from 10 W to 110 W and it affects levels of the randomness (disorder) of the samples. The room temperature resistivity of our films range from 15 µΩcm to 95 µΩcm.
Here we would discuss a little bit the operating principles of sputtering. Sput-tering is a technique used to deposit thin films of a material onto a substrate. By first creating a gaseous plasma and then accelerating the ions from this plasma into a source target, the source material is eroded by the arriving ions via energy transfer and is ejected in the form of neutral particles. As these neutral particles are ejected they will travel in a straight line unless they come into contact with other particles or a nearby surface. If a substrate is placed in the path of these ejected particles it will be coated by a thin film of the source material.
The ”diode sputtering” example given above has proven to be a useful tech-nique in the deposition of thin films when the cathode is covered with sputtering target. Diode sputtering however has two major problems. First one is the depo-sition rate is slow and second one is the electron bombardment of the substrate is extensive and can cause overheating and structural damage.
The development of magnetron sputtering deals with both of these issues simultaneously. Figure 4.3 shows the schematic of the magnetron sputtering. By
4.2 Sample Preparation
Figure 4.2: Processes of deposition by using photolithography.
4.3 Low Temperature Resistance and Magnetoresistance Measurement
using magnets behind the cathode to trap the free electrons in a magnetic field directly above the target surface, these electrons are not free to bombard the substrate to the same extent as with diode sputtering. At the same time the extensive, circuitous path carved by these same electrons when trapped in the magnetic field, enhances their probability of ionizing a neutral gas molecule by several orders of magnitude. This increase in available ions significantly increases the rate at which target material is eroded and subsequently deposited onto the substrate.
The ensuring process might be compared to a find sand blasting in which the momentum of the bombarding particles is more important than their energy. The inserted argon gas is chosen because it is a heavy rare gas and is plentiful. It also has a low ionization potential.
Figure 4.4shows the film by using photolithography. The width of the film is 50 µm. Silver plaster is used to stick four copper wires with diameter 50µm on the four electrodes with 0.5 mm square.
The talbe 4.1 shows the sputtering parameters for all films. The pressure is the chamber pressure during deposition and thinkness is the deposed thinkness of films. In the system, we use two kinds of mask, metal mask and photomask.
The deposition rates are basically proportion to the sputtering powers.