DEPOSITION OF THIN TiO 2 FILMS BY DC MAGNETRON SPUTTERING METHOD

Transcription

1 Chapter 4 DEPOSITION OF THIN TiO 2 FILMS BY DC MAGNETRON SPUTTERING METHOD 4.1 INTRODUCTION Sputter deposition process is another old technique being used in modern semiconductor industries. Sputtering is a process that can deposit TiO 2 material on wafer/glass substrates. In this process target is connected to negative high voltage. Further argon gas is introduced into the chamber and is ionized to a positive charge. The positively charged argon atoms are attracted and strike the negatively biased target. The sputtered atoms and molecules scatter in the chamber and out of which some comes to rest on the wafer. A principal feature of a sputtering process is that the target material is deposited on the wafer without chemical or compositional change. In the present work, DC sputtering techniques has been used: 4.2 DC SPUTTERING In DC sputtering the target is connected to a negative potential with a positively charged anode present in the chamber as shown in Figure-4.1. Figure 4.1: DC sputtering system 76

2 The negatively charged target ejects electrons, which are accelerated toward the anode. Along the way they collide with the argon atoms, causing their ionization. In order to initiating the sputtering process, the positively charged argon atoms are accelerated to target material. A secondary effect of the ionization process is the impact of the electrons on the gas atoms, resulting in the plasma visible as the glowing purplish region just in front of the target. Dark spaces exist just in front and to the sides of the target. Sputtering efficiency is enhanced when plasma is confined to the region between the target and wafers. The shields can be used to prevent target material from being sputtered from the sides, material that will never deposit on the wafers. 4.3 FACTORS INFLUENCING THE DEPOSITION RATES There are various parameters which can affect the deposition rates i.e currentvoltage, pressure in chamber and the ground shielding. In the energy range employed for the deposition of most films, the sputtering yields increases relatively slowly with increasing ion energy or applied voltage. On the other hand the number of particles striking the cathode is proportional to the current density. Consequently, current is a significantly more important parameter for determining the deposition rate rather than voltage. As the pressure in a sputtering system is raised, the ion density and, therefore, the sputtering-current density increase. Therefore, it is not surprising that the deposition rate increases linearly with pressure provided the latter is not so high. At higher pressure deposition rate becomes less. The reason for reduction in deposition rate is that the material sputtered from the target may collide with gas atoms on its way to the substrate, at a rate, which will increase with increasing pressure. The result of the collision is to deflect the sputtered atom, sometimes back toward its parent, and hence deposition rate is decreased. Below pressures of about 20 mtorr back diffusion is negligible. It is only at pressures in excess of approximately 130 mtorr that more than half the material that leaves the cathode returns to it through diffusion. With increasing pressure, deposition becomes less a line-of-sight process and more a diffusion process. So as expected, deposition rate falls off rapidly with increasing pressure. 77

3 In fact the optimum pressure to use for sputtering may be lower than 130 mtorr. This is because of the problem of backscattering and also because the reduced size of Crookes dark space at higher pressures. In sputtering systems the target is surrounded by dark space shield, also known as a ground shield. The purpose of this is to restrict ion bombardment and sputtering to the targets only. Otherwise, the target backing plate, mounting clips and mechanical supports would also be sputtered and cause the film to be contaminated. In order to prevent ion bombardment of the protected regions, the space between the target and ground shield must be less than the thickness of the dark space. Since the thickness of the dark space decreases with pressure, the size of the gap between the target and shield sets as an upper pressure limit for operating the system. Following are the advantages of sputter deposition technique over other deposition techniques. Sputter deposition technique is useful for deposition of complicated materials such as stainless steel without change of composition provided target temperature is kept low. Sputtering can be accomplished from large area targets. This often simplifies the problem of film thickness uniformity and shadowing effect also becomes less pronounced. Cleaning of substrates is much simplified because the surface of substrates can be sputter cleaned by ion bombardment before sputter deposition. Shutters between target and substrates are often very useful or necessary for pre-sputtering in order to achieve thermal or background pressure-equilibrium conditions. Through DC or RF sputtering of metals or semiconductors in reactive gases such as oxygen or nitrogen, it is possible to form oxidized or nitrided films of materials such as SiO 2, TiO 2, Si 3 N 4, and SnO 2. Film thickness control becomes relatively simple. First, the deposition rate by laying down a film of a thickness that will be easy to measure is determined, and thereafter with the same geometric arrangements and under the same operating conditions (gas pressure, target current, and voltage) it becomes largely a matter of adjusting the deposition time to achieve film thickness. 78

4 The presence of the plasma offers other unique possibilities for achieving films with desired properties. Negative biasing of the substrates before film deposition can be used to remove oxide films and to improve film adherence in case of metal films on metal substrates. Including these main disadvantages of sputtering is its relatively low deposition rate. Typically deposition rates are in the range of 50 to 400 A min -1. For higher sputtering rates, it is necessary to cool the target. If higher sputtering rates are desired by increasing the plasma density, then it becomes necessary to find the means for efficient substrate cooling. In this study TiO2 thin films were deposited successfully by DC Sputtering method. This work is aimed at the study of the influence of process parameters like deposition rate, substrate temperature and annealing temperature on the electrical properties like maximum capacitance, dielectric constant, fixed charge, interface trapped charge and leakage current. For making this analysis we have used p-type single crystal silicon <100> as substrates and employed direct current (DC) magnetron sputtering method with Titanium metal as target and Oxygen as reactive gas. TiO2 thin films have been deposited with an expected thickness of 50 nm with different deposition rates starting from 0.5 nm/minute to 5 nm/minute with different substrate temperatures (ambient temperature to 550ºC). Some of the samples are annealed at 750ºC in oxygen atmosphere for 30 minutes and 950ºC in nitrogen ambient. In order to deposit thin high k films on Si, first step is to ultra clean the wafer with some standard method. 4.4 CLEANING PROCEDURE In the present work P-type (100) Si substrates were subjected to ultrasonication for removing the contaminants like dust particles etc., cleaned in a dilute HF solution (HF: H 2 O = 1:100 by volume) for 10 minutes to remove any organic residues left on the substrate and to make it hydrophilic. Then the samples were rinsed in De Ionized (DI) water and then dipped in 5% HF solution to remove any native oxide (SiO2) present on the samples. Finally the samples are rinsed in DI water and then flushed with dry Nitrogen prior to loading in the deposition chamber. 79

5 4.5 DEPOSITION OF TITANIUM BY DC MAGNETRON SPUTTERING TECHNIQUE TiO 2 layer of nearly 30nm thickness was deposited by sputtering techniques using a DC sputter system in the Electronic Science Department of Kurukshetra University. The target used was of 5-inch diameter disc with 99.99% purity. The substrate holder shown in Figure-4.2 was 7 inch below the target and was watercooled. Prior to sputtering, the system was allowed to pump down to a base pressure of the order of 10-6 torr using an oil diffusion pump and a rotary pump. The system was flushed with argon gas of 99.99% purity. Figure 4.2: Substrate Holder The pressure during deposition was 20 mtorr. Sputtering was then carried out by applying a negative DC voltage of 2 KV to the target. The thickness of TiO 2 layer was controlled by selecting the power, pressure and time from the calibration curves obtained by a series of deposition experiments carried out prior to the main deposition. 80

6 The samples under investigation were deposited onto p-type (100) single crystalline silicon substrates in the 1 10 Ω cm resistivity range. TiO 2 films were deposited by DC reactive sputtering of a titanium (Ti) target, in mixed argon (Ar) and oxygen atmosphere. The experiment was carried out using deposition system as shown below in Figure-4.3. Pure Titanium (99.997%) of 76 mm diameter and 33 mm thickness has been used as a sputtering target. Argon (Ar) gas (99.996% pure) and oxygen O 2 (99.99 pure) were used as the sputtering and reactive gases respectively. Figure 4.3: Sputtering system used in lab The flow Rate of Ar was maintained constant at 50 standard centimeters cubic per minute (sccm) while flow rate of oxygen was varied at kept at 7 sccm, 10 sccm and 81

7 15 sccm for different composition of the film. The total pressure during the deposition in the chamber was kept at 100 mtorr. The sputtering power was kept at 90, 120 and 150 watt. A rotary and diffusion pump combination was used to get the required vacuum. After attaining the base temp of 10-5 mbar, keeping the oxygen pressure to a set value, argon was let in to achieve the desired pressure. After deposition samples were cooled down to room temperature. In order to induce their crystallization, a subsequent post deposition annealing (PDA) process was carried out at 950 C in oxygen atmosphere for 30 minutes. 4.6 FORMATION OF HIGH k MOS STRUCTURE Thermal evaporation is used for the deposition of metals on discrete devices and circuits of lower integration levels. Vacuum evaporation takes place inside an evacuated chamber. The chamber was of a stainless steel enclosure. Inside the chamber, there is a mechanism to evaporate the metal source. It contains a wafer holder, a shutter, and a heater. For preparing Al/TiO 2 structure with evaporation technique, Al was evaporated through a metal mask having circular openings of 2 mm diameter. The mask is placed in close proximity to the substrate, thereby allowing condensation of the evaporant vapour only in the exposed substrate areas. The evaporation was done by resistive heating of Al in a vacuum evaporation system. For achieving a good quality metal deposition with proper adhesiveness, it is essential that environment in the vacuum deposition chamber must be clean. The chamber was evacuated to a pressure of 10-6 torr prior to the metal deposition. A series of depositions were carried out to obtain the calibration curves for thickness versus amount of copper material for a fixed evaporation source geometry. 4.7 CONCLUSION The thin near stoichiometric films has been deposited successfully by DC magnetron sputtering method at various oxygen flow rate, various substrate and annealing temperature. The variation in parameters was applied to confirm the influence on optical and electrical properties on the deposited samples. Which have been characterized in chapter 5. 82