Thin Film Substrate
Question: How should I store lithium evaporation pieces?
Answer: To avoid oxidation, lithium should be kept either in oil or an argon inert atmosphere. Unused pieces should be repackaged in the oil with a tight seal and stored in a cool, dry place.
Question: How do I clean oil-covered evaporation pieces?
Answer: Evaporation pieces that react with air or water vapor are protected during storage and shipment by immersion in a low viscosity (hydrocarbon) mineral oil. Before the material can be installed in a vacuum system, this oil must be completely removed using solvents that do not act as additional contamination sources for the vacuum system or the subsequent thin-film processes.
Question: How do I thermally evaporate silicon dioxide?
Answer: We recommend heating the substrate to 350°C before attempting to thermally evaporate silicon dioxide. We anticipate a deposition rate of 2 angstroms per second when the evaporation temperature is at ~1,200°C. A partial pressure of O2 at 1-2 X 10-4 Torr is recommended. Under these parameters, we anticipate films to be smooth and amorphous. The material should be replaced when it becomes dark or black.
Thermal evaporation of silicon dioxide is generally not done due to the difficulty associated with this method. The simplest approach would be to use a relatively inexpensive boat source and change the material as often as possible. We recommend starting with a thick gauge, tungsten boat. It is possible to use a shielded, tantalum crucible heater with an aluminum oxide crucible. However, this method may be unsuccessful because tantalum baffle boxes cannot withstand high temperatures. In order for silicon dioxide to sublime and evaporate, the temperature of the baffle box must be between 1,500°C and 1,800°C. Once the material’s temperature is within this range, there is potential for the material to alloy with the box, causing it to fail. Silicon dioxide mimics silicon when in the melted state.
If using the baffle box method, great care must be taken when installing the heater to prevent the outer shields from becoming warped which can cause a short in the heater, causing the welded joints to fail. The heater should be centered between the contacts and the outer shielding must be clear of the leads.
Another option would be reactive evaporation. Silicon monoxide (SiO) can be placed in a tantalum baffle box with a substantial amount of oxygen (we recommend adding 1-2 X 10-4 Torr). We have not encountered any problems thermally evaporating silicon monoxide. However, it is necessary to replace the material after every run. Silicon monoxide is hard to convert to silicon dioxide because the bond energy for silicon monoxide is higher than that for silicon dioxide. As with silicon dioxide, the temperature of the baffle box must be between 1,500°C and 1,800°C in order for evaporation to take place. Once the material’s temperature is within this range, there is potential for the material to alloy with the box, causing it to fail. Silicon monoxide also mimics silicon when in the melted state.
Question: How do I thermally evaporate titanium?
Answer: Titanium can be thermally evaporated using a thin width, thick gauge, high current tungsten boat or a shielded tantalum crucible heater with a tall, intermetallic crucible. However, there is risk of failure with either option.
It is important to note that titanium alloys with refractory metals. A material’s evaporation temperature is often regarded as that needed for the material’s equilibrium vapor pressure to be 1E-2 Torr. At that vapor pressure, the deposition rate on a substrate in a system of “normal” geometry is good or high. For titanium, that temperature is ~1,750C. Titanium has to melt and “wet” a boat or crucible in order for efficient evaporation to take place. At this temperature, titanium will be liquid and quickly alloy with a refractory boat, destroying its electrical and mechanical properties. The end result is the boat cracking and falling apart. Despite this, we have had limited success thermally evaporating titanium from a thin width, thick gauge, high current, tungsten boat.
A second option for thermal evaporation is to use a shielded tantalum crucible heater with a tall, intermetallic crucible. Thin films of titanium can be evaporated from intermetallic crucibles. However, film thickness may be limited to 500 angstroms, and the crucible may need to be replaced for each subsequent run. Intermetallic crucibles are composed of titanium boride (TiB2) and boron nitride (BN). This material combination works well because the material is both lubricious and electrically conductive. The crucible is both strong and conductive, yet its lubricious properties help prevent material spill-over and crucible cracking. Great care must be taken when installing the heater to prevent the outer shields from becoming warped which can cause a short in the heater, resulting in failure of the welded joints. The heater should be centered between the contacts and the outer shielding must be clear of the leads.
Using the heater/crucible set-up involves heat transfer by thermal radiation across the interfaces (heater-crucible exterior and crucible interior-evaporant surface) and by conductivity through the crucible and evaporant. For titanium to reach proper evaporation temperature, the heater and crucible must be higher (in some instances, much higher) in temperature. A significant portion of that power is simply radiated to the cold chamber walls. Customers typically observe high input powers and high thermal loads on the substrate.
While some customers have tried the heater/crucible method for thermally evaporating titanium, it is possible that the set-up could fail. The heater/crucible may not get hot enough to melt and evaporate the titanium. Overfilling the crucible can also be detrimental to the process. The titanium could creep out over the walls of the crucible and react with the heater causind it to fall
One downside of the header/crucilble set-up is that liquid titanium is a universal solvent.In other words,it reacts with and destroys almost all crucible materials.Titanium,once diffused through the crucible,can attack the heater as well.
The heater/crucible option is more cost-prohibitive than the tungsten boat option. It could fail during the first run or even before if the material does not reach the proper temperature for evaporation.Crucibles should be stored in a cool,dry place and always handled with gloves or forceps.
Question: How do I thermally evaporate germanium?
Answer: In a deposition system with normal dimensions, a material with an equilibrium vapor pressure (EVP) of 1E-2 Torr will deposit at a rapid rate. Therefore, 1E-2 Torr is typically regarded as the EVP one should attempt to achieve during deposition. The only parameter that affects EVP is temperature. We reference various charts or books to determine what temperature is needed to make the material’s EVP 1E-2 Torr. Comparing that temperature with the material’s melting point indicates whether the material will evaporate or sublime at that vapor pressure.
Germanium’s melting point (937°C) is well below the temperature to reach 1E-2 Torr. Hence, germanium evaporates as opposed to subliming. We recommend that the base pressure for germanium evaporation should be 10-6 Torr or lower with an evaporation temperature of 1,400°C. The average deposition rate under these parameters is 1-5 angstroms/sec.
Germanium can be thermally evaporated using a tantalum or tungsten dimple-style boat. It is not known to alloy with refractory metals.
Question: How do I thermally evaporate nickel/iron (81/19 WT%) alloys?
Answer: Thermal evaporation is very difficult to accomplish with nickel/iron alloys.
Nickel and iron each have a vapor pressure of ~1E-2 Torr at around 1,500°C with nickel’s vapor pressure being slightly lower at any given temperature. When the difference in atomic percentage is factored in, deposited films are expected to have a composition that is close to the 81/19 alloy.
Both elements react with refractory metals when in liquid form, thus making evaporation out of a tungsten boat nearly impossible. Even when using a thin width, thick gauge tungsten boat, the alloy will vigorously attack the boat, causing it to crack and break. Therefore, the boat may not even last one run. Also, a tungsten boat may not work because there is risk that the materials may alloy with the tungsten, changing the mechanical and electrical properties of the boat. Due to differences between their melting points, the nickel and iron may briefly phase separate during evaporation. Because iron has a slightly higher vapor pressure at the same temperature as nickel, films may initially be iron rich.
An alternative option is to evaporate out of an aluminum oxide crucible. However, this method may also be unsuccessful. The crucible may only last for one run due to differences in the coefficients of expansion between the alloy and the aluminum oxide crucible as the melt cools. Because the material rests at the bottom of the crucible and, typically, the crucible’s bottom is cooler than its sides, the heater and crucible must be heated to a much higher temperature to achieve evaporation. A power supply may not be capable of getting the heater hot enough. Also, at 1,500°C, the vapor pressure of the aluminum oxide and, possibly even the tungsten, may cause film contamination.
Question: How do I evaporate indium?
Answer: Indium can be deposited via e-beam or thermal evaporation. However, it is rated excellent for e-beam evaporation.
When depositing any material in a vacuum system, there should be a lot of consideration taken upfront. Indium is not particularly problematic; however, any residual indium in the system will vaporize if it reaches a high enough temperature during subsequent deposition runs. It is recommended to use adequate foil shielding which can be removed after deposition is complete.
Indium can be thermally evaporated out of a tungsten basket heater with an aluminum oxide crucible. Overspray can be limited to some degree by reducing the amount of material loaded in the crucible. Crucibles should be stored in a cool, dry place and always handled with gloves or forceps.
Indium can be e-beam evaporated from a Fabmate®, graphite, or molybdenum crucible liner and is rated excellent for e-beam evaporation. A key process note is to consider the fill volume in the e-beam application because we find that the melt level of a material in a crucible directly affects the success of the crucible liner. Overfilling the crucible will cause the material to spill over and create an electrical short between the liner and the hearth. The outcome is cracking in the crucible. This is the most common cause of crucible liner failure. Placing too little material in the crucible or allowing the melt level to get too low can be detrimental to the process as well. When the melt level is below 30%, the e-beam is likely to strike the bottom or walls of the crucible which immediately results in breakage. Our recommendation is to fill the crucible between ⅔ and ¾ full to prevent these difficulties.