Thin Film Deposition Techniques & Capabilities

The basic configuration typically includes an electron beam source, hearth, and top cover, with accessory feedthroughs and appropriately sized power supplies. Evaporation can be done directly from the hearth or with a crucible liner depending on the intended evaporation material.

Because of the ability to control deposition rate, low contamination, excellent material utilization, and very high deposition rates, electron beam evaporation is a very adaptable technique used in many thin film deposition applications.

Similar to other techniques, the chamber pressure is brought to as low a level as possible to prevent background gases from chemically reacting with the film or bulk evaporant. Under carefully controlled partial pressures of reactive gases, reactive thermal evaporation can create films of a different chemical composition than that of the bulk material.

The basic configuration typically includes a boat, filament, or crucible source with accessory feedthroughs (water-cooled if necessary), cross-contamination shielding, shutters, and appropriately sized power supplies.

A voltage is applied between the target material (cathode) and the substrate (anode) to be coated with the target material. Initial electrons from the target's surface cause cascade ionization in the chosen process gas and thus, plasma is formed. Sputtering sources are compatible with vacuum levels from a few mTorr to UHV and come in many shapes and sizes, from small 1" round type R&D cathodes to large planar production cathodes.

Because the plasma is both electrically neutral and highly conductive, there is little voltage drop across it. The voltage drop occurs across thin "dark space" regions (areas between the plasma and each electrode). The target's negative potential attracts positive ions from the plasma's edge, which in turn hit the target with sufficient kinetic energies to eject neutral target atoms/molecules by energy transfer. While traveling from target to substrate, each ejected atom hits numerous gas atoms/molecules that deflect them and cause energy loss. By optimizing the target-substrate distance, the atoms approach the substrate's surface from partially randomized directions, producing a uniform film thickness across a textured substrate's surface.

For circular sources, the optimum throw distance between target and substrate is larger than the target's diameter to "smooth out" the source's ring-like deposition pattern. By contrast, a linear production source used to coat large area substrates moving across it has a much shorter optimum throw distance.

Similar to other techniques, the chamber pressure is brought to as low a level as possible to prevent background gases from chemically reacting with the film or sputter target. Under carefully controlled partial pressures of reactive gases, reactive sputtering can create films of a different chemical composition than that of the bulk material.

The basic configuration typically includes: a TORUS^® sputtering source with a target of the desired coating material; shutters; deposition chimney and/or gas injection; and appropriately sized DC, Pulsed DC, and RF power supplies.

The evaporation temperatures of organic materials are low compared to that of most metals, typically much less than 500°C, and the evaporation rate is exceptionally sensitive to the material's temperature. To achieve satisfactory film deposition demands rigorous temperature control. For this purpose, low temperature evaporation sources designed specifically for depositing organic thin films are frequently used in sequential and co-deposition applications.

Similar to other techniques, the chamber pressure is brought to as low a level as possible to prevent background gases from chemically reacting with the film or bulk evaporant. Under carefully controlled partial pressures of reactive gases, reactive thermal evaporation can create films of a different chemical composition than that of the bulk material.

The use of a glove box to control the ambient atmosphere for loading and unloading organic evaporants is a very useful tool for this technique. Organic compounds are typically volatile and reactive in atmospheric conditions and at room temperature. Glove box integration allows creation of an inert environment to keep and control the evaporant's native properties.

The basic configuration typically includes a point source with accessory feedthroughs, cross-contamination shielding, shutters, and appropriately sized power supplies and PID controllers.

During each pulse step, chemical reactions between precursor molecules and active surface species yield new surface species that passivate the surface. Once the surface becomes fully passivated, reactions are complete and result in the formation of a limited number of new surface species. Uniformity depends primarily on the distribution of active surface species/sites and completion of surface reactions during each precursor pulse step. Subsequently, remaining precursor and/or reaction by-products are purged in preparation for the next pulse step. A complete cycle is required to obtain the desired material. Each cycle deposits a very specific amount of material onto the substrate surface and is repeated until the desired amount of material has been deposited, enabling very accurate control of film thickness.

Thermal-based methods depend on substrate heating where the range of process temperatures resulting in ideal growth is referred to as the "ALD window." Ideal growth is characterized by chemical adsorption of surface species that is irreversible, self-limiting, and complete. Outside of the ALD temperature window growth becomes non-ideal.

Plasma-Enhanced ALD (PEALD) methods utilize reactive plasma species as precursors for ALD surface reactions. Typical plasma gases include O2, N2, and H2. Benefits of PEALD include lower temperature process capability as well as new pathways for chemical reactions that would otherwise be inaccessible by purely thermal methods. Plasma treatments can also be used for substrate surface modification prior to ALD processing.

The basic configuration typically includes a sealed reaction chamber with flow characteristics suitable for entry, exit of reactive precursors, multiple precursor delivery modules, and high-speed valving and software control.

Rather than different techniques, some applications need multiple examples of the same technique. This is particularly true and very common with sputtering, where an array of cathodes and/or effusion cells for organic materials may be mounted in one chamber.

One critical aspect of multiple or combined techniques in one deposition system is cross-contamination. The "plume" of one material must not deposit on (contaminate) the material in another deposition source. This is achieved by careful design of sources, shields, and shutters.

Another important aspect of multi-chamber systems is source orientation geometry. Evaporation sources are mounted with the vapor plume's axis vertically up. This avoids spillage if/when the material is molten at its evaporation temperature. Magnetron sputtering cathodes are not orientation dependent, i.e., they can be mounted at any angle. Two common arrangements for multiple sputter sources are "parallel" and "convergent." Parallel orientation is a common arrangement in large area coaters where a number of substrates are mounted on a rotating platen, and the platen moves to locate a substrate's center over each source's center in turn (this arrangement suits sequential layer deposition). Convergent sources have axes that meet at a point in space that coincides with the substrate surface's center-point, or offset slightly to achieve better uniformity. This arrangement is particularly useful when co-depositing different materials on a single substrate.

Multi-Chamber deposition systems can also be accommodated by combining chambers/modules that house different techniques. This arrangement typically avoids cross-contamination quite well. More complex combinations interconnect UHV chambers for, perhaps, MBE deposition and surface science analysis, or high vacuum chambers for plasma etching, metal deposition, organic deposition, mask storage, ALD, etc., in a cluster arrangement.

In contrast to typical arc sources, the filtered arc source allows separation of macroparticles from the plasma. Cathodic arc sputtering condenses ions, which offers advantages over standard magnetron sputtering for some applications due to the generation of a plasma of sputtered material rather than neutrals created in standard sputtering. This technique is useful for sputtering graphite into diamond-like carbon with a greater level of film property control than is possible with traditional cathodic arc deposition, and has also been used with silver dopant for creating antibacterial properties. Substrate biasing allows control of the arriving atoms' energy and tuning of the film's compressive stress. This technique also operates at lower pressures than traditional sputtering.

In such a discharge, the target peak power densities during the pulse can be in the order of several tens of kilowatts per square centimeter whereas the average power densities are comparable or equal to that of Direct Current, DC, Magnetron Sputtering. Of particular note is that to achieve such high power densities on a target one only needs to upgrade their power supply. The high peak power pulses on the target in HIPIMS result in plasma electron densities up to 10^19 m-3 which is three orders of magnitude higher compared to DC Sputtering. These high plasma densities promote ionization of the sputtered material forming an ionized sputtered material flux, where the ionization fraction can reach 90%. The ion flux is subject to electric and magnetic force so its direction and energy can be controlled and be used to perform substrate pre-treatment as well as to enhance film and device properties.

The KJLC wedge tool can help deposited films of material with n number of variations in addition to shadow mask. n is number of columns and its width can be programmed.

Example: Six step (or six columns) deposition of various thickness of a "Material" or six deposition of different materials of same thickness or various thicknesses

The substrate can be ROTATED 90, 180 or 270 degrees in the cassette and repeat the above wedge operation.

Using the KJLC dual wedge tool in combination with the rotation/orientation of the substrate will result in nxn depositions. In example given a total of 36 can be prepared with straightforward operation of the dual wedge and substrate rotation.

Reactive sputtering can be used with a base metal target (e.g., titanium plus oxygen gas) or to control the stoichiometry of the film from a conductive compound/ceramic target (e.g., ITO plus oxygen gas). The reactive gas partial pressure required to form a particular stoichiometric compound on the substrate usually causes the surface of the target to be partially reacted. This produces areas on the target surface with different electrical properties, resulting in varying charge accumulation and arc formation to dissipate the charge difference. These arcs are strong enough to cause local evaporation resulting in the undesirable ejection of macroparticles, unstable process parameters, and possible target damage.

KJLC utilizes our bipolar pulsed DC supply to mitigate arc formation. Instead of applying a constant negative DC voltage to the target, the potential is reversed to a positive voltage (15% of the negative voltage magnitude) for a short duration, many thousands of times a second. This positive pulse draws an electron current from the plasma and neutralizes charge build-up on the target surface, whereas the longer period of applied negative voltage sputters the target.

The duration and frequency of the positive voltage reversal are fully controllable from 1-10µs (1µs resolution) and 2-100 kHz, respectively. The full range of duty cycle allows a user to set the appropriate pulsing parameters to outpace charge build-up creating a stable reactive sputtering process while maximizing the deposition rate.

It is important to note that the DC pulse frequency is high enough to maintain a stable reactive deposition process but is not as high as the 13.56 MHz typically used in Radio Frequency (RF) processes. Pulsed DC provides advantages over RF as it does not require complicated matching networks, offers higher deposition rates, and typically has better arc control.

KJLC has pulsed DC supplies with 1 kW and 2 kW maximum single-output power, which are a good fit for 2" to 4" magnetron sputter cathodes. Figure 1 is our recommended size of power supply for the size of the sputter cathode. Additionally, the power supplies have a suitably large current range for reactive sputtering processes at 2.5 amps and 5 amps, respectively.

Figure 1: Recommend power supply for each sputter cathode size.

When some target materials are exposed to a reactive gas, their emission of secondary electrons increases, and the voltage required to sustain the target plasma reduces. As the voltage decreases, the current must increase to maintain a fixed power. Furthermore, it is sometimes beneficial to choose pulsing parameters that operate the target at the lowest voltage and highest current. Sputtering at lower voltages is especially useful when working with complex oxides, such as ITO, to reduce high energy neutral bombardment on the growing thin film. The high energy neutral bombardment can be detrimental by causing preferential re-sputtering of the film, driving it non-stoichiometric. As for the voltage output of these supplies, both the 1 kW and 2 kW units can output up to 800 V, which is well suited to ignite and sustain a plasma at standard operating pressures.

Nitrogen gas (N₂) is commonly used for making reactively sputtered nitride films, although it is only moderately chemically reactive in its diatomic form. It may be beneficial to increase its reactivity by applying RF bias on the substrate while doing a pulsed DC sputtering process on the sputter cathode. The RF bias creates a plasma to crack the nitrogen into monoatomic form and adds energy to promote the reaction. KJLC has rigorously tested the pulsed DC supplies and verified stable operation while RF bias is applied to the substrate.