Ion Assisted Deposition of Thin Films

Deposition of thin films by thermal and electron beam (e-beam) evaporation processes was discussed in the previous two Blogs. One of the major problems with these processes is the low energy of the evaporated atoms, which can cause problems in the thin films such as poor adhesion to the substrate, reduced density, porous and columnar microstructure, increased water pick up and poor mechanical properties. Typically, the substrate is heated to several hundred degrees Celsius during coating to mitigate this effect, but it is by no means eliminated. Ion assisted deposition (IAD) and ion plating mitigate many of these problems by providing enhanced energy to the evaporated atoms and ion cleaning the substrate [1].

For optical coatings, problem with porous films is that they can subsequently absorb moisture, which changes the refractive index of the layer(s), and can cause shifts in the center wavelength with changes in ambient temperature and humidity.  Low density also limits mechanical durability to some extent, although these films can typically meet most of the MIL-SPEC durability and environmental requirements.  Furthermore, the requirement to heat the components during processing can limit substrate material choice, and also introduce stress in the substrate due to thermal cycling.

Figure 1 below shows the placement of an ion source in a typical deposition chamber. Bombardment prior to deposition is used to sputter clean the substrate surface. Bombardment during the initial deposition phase can modify the nucleation behavior of the depositing material such as nucleation density. During deposition the bombardment is used to modify and control the morphology and properties of the depositing film such as film stress and density. It is important, for best results, that the bombardment be continuous between the cleaning and the deposition portions of the process in order to maintain an atomically clean interface [1]. IAD adds a high energy ion beam that is directed at the part to be coated.  These ions impart energy to the deposited material, acting almost like an atomic sized hammer, resulting in a higher film density than achieved with purely evaporative methods.

The higher density of IAD coatings generally gives them more mechanical durability, greater environmental stability and lower scatter than conventional evaporated films.  Furthermore, the amount of energetic bombardment can be varied from zero to a maximum level on a layer by layer basis, giving the process tremendous flexibility.  For example, while IAD is not compatible with some of the commonly used materials in the infrared, it can be used solely on the outermost layer to yield an overall coating with superior durability.  The ion energy can also be used to modify the intrinsic stress of a film during deposition.  In some cases, this can change the film stress from tensile to compressive, which can help to maintain substrate surface figure, especially when depositing thick infrared coatings.

Several types of ion source are available, including Kaufman type, end Hall type, cold and hollow cathode types. Each source differs in how ions are created and accelerated to the substrate and ion energy distribution. All these sources ionize inert gases such as argon and krypton to bombard the substrate. Ions are extracted by several methods, grid extraction (Kaufman type) where ions are relatively monoenergetic or from a broad beam ion source (end Hall and hollow cathode) having a spectrum of ion energies. Figures 2 and 3 show cross sections of Kaufman and end Hall ion sources [1,2,3]. The Kaufman source is gridded to control energy of exiting ions. Ion current available from the ion source are determined by source parameters, such as gas pressure, cathode power, anode potential, geometry, etc. The accelerator grid serves two purposes: 1) to extract the ions from the discharge chamber, and 2) to determine the ions trajectories, i.e. focusing. Table 1 shows operating parameters for this ion source.

Table 1. Maximum argon ion beam current

Discharge voltages for end Hall ion sources typically range from 40 – 300V. Discharge currents for hollow cathode designs range from 30 μA – 5A with discharge voltages up to 16 kV [4]. This unit is also used for ion etching and ion sputtering.

A wide range of properties of evaporated (and other PVD processes as will) coatings improve with increased density and packing density resulting from ion bombardment. Effects of IAD are no more apparent than in moisture stability and stress in optical coatings [2]. Moisture can readily penetrate low density films deposited using low energy processes, causing a distinct shift in optical properties (refractive index, absorption), as shown in Figure 4. We see that moisture shift in the refractive index is significant for as deposited HfO2 films while negligible for IAD films.

Stress in evaporated thin films deposited without IAD is generally tensile. Compressive stress is desirable in thin films to enhance mechanical properties and reduce cracking, although all stress should be kept to a minimum. By removing loosely bound atoms IAD increases film density and can change stress from tensile to compressive. The capability to tune stress is particularly valuable in multilayer films. Thus it is possible to vary stress of alternating layers from tensile to compressive, thus achieving very low stress in the resulting structure.

However, with IAD there can be too much of good thing and defects can form during deposition. As a result of this process ion energy can be given up to the growing layer either at the surface of in the underlying regions [5]. Atom displacements responsible for lattice damage (voids, lattice modification) are produced by energy deposition in bulk regions of the film. Contrast this with surface smoothing described previously. Additionally, inert gases can also be driven into the film. An extension of this effect is ion implantation used extensively in semiconductor technology. Fortunately, the threshold energy needed for bulk defect formation is higher than the threshold for surface driven processes. Care must thus be taken not to use excessive ion energy or bulk defects will be created.

Ion sources are also used for ion beam sputter deposition of thin films, but that will be addressed in a future Blog.

Figure 1. Ion source placement in deposition chamber.

Figure 2. Kaufman ion source geometry [3]
Figure 3. End Hall ion source geometry [1]
Shift in refractive index for evaporated HfO2 films with and without IAD [2]
Reference:

  1. Donald M. Mattox, in Handbook Deposition Technologies for Films and Coatings, 3rd Ed., P M Martin Ed., Elsevier (2009).
  2. D. E. Morton, V. Fridman, 41st Annual Technical Conference Proceedings of the SVC, 297 (1998).
  3. South Bay Technology, Applications Laboratory Report 123.
  4. J Alessi et al., Brookhaven Laboratory Report 102494 -2013 CP.
  5. A R Gonzalex-Elipe, F Yubero & J M Sanz. Low Energy Ion Assisted Film Growth, Imperial College Press (2003).

 

AI & Virtual Reality—Let The Games (and Work) Begin!

Artificial Intelligence (AI) and Virtual Reality (VR) have enormous potential for entrepreneurs, designers, software experts, video gaming developers and high-tech manufacturers. Some common AI and VR essentials include very powerful computing power in extremely small/tiny spaces and exceptionally high definition displays—real or projected. There will be a requirement for many sensors and actuators too. If today’s marketplace prices are any indication, consumers, businesses and militaries will pay handsomely for the very best. But that presents several challenges including terminology.

AI terminology seems to be described somewhat uniformly but what a given company describes as AI can vary enormously. VR has several common descriptive variants since enhanced reality is more affordable to manufacture and more affordable for consumers. In recent announcements, Microsoft has announced Windows Mixed Reality headsets as part of a new thrust, “We are on a mission to help empower every person and organization on the planet to achieve more, and one of the ways we are doing that is through the power of mixed reality,” said Alex Kipman, Technical Fellow at Microsoft. This is a follow to their earlier HoloLens headsets. The technology behind it will allow its flagship operating system to use the latest generation of Windows 10 hardware devices and software for augmented and virtual reality technologies experiences.

Caption Windows 10 Mixed Reality headsets from partners Lenovo, Acer, Dell & HP

 

Google’s Alphabet’s X division has moved on to refining their technology with the Google Glass 2.0 headsets the result. Google is targeting business and manufacturing applications that will help boost productivity. The original Google Glass headsets had some appeal but some problems that were addressed for the version 2.0 headsets.

Caption: Google Glass 2.0 headset

 

Augmented Reality

“AR is a live direct or indirect view of a physical, real-world environment whose elements are “augmented” by computer-generated or extracted real-world sensory input such as sound, video, graphics or GPS data. It is related to a more general concept called computer-mediated reality, in which a view of reality is modified (possibly even diminished rather than augmented) by a computer. Augmented reality enhances one’s current perception of reality, whereas in contrast, virtual reality replaces the real world with a simulated one.[1][2] Augmentation techniques are typically performed in real time and in semantic context with environmental elements, such as overlaying supplemental information like scores over a live video feed of a sporting event,” according to Wikipedia. [https://en.wikipedia.org/wiki/Augmented_reality]

Apple is excited about Augmented Reality and will soon introduce such capabilities soon on iPhones and iPads. There are rumors that they may intro glasses too. What is for real there is Apple’s ARKit, a set of software developer tools for creating augmented reality apps or iOS.

According to Apple developers, “Apps can use Apple’s augmented reality (AR) technology, ARKit, to deliver immersive, engaging experiences that seamlessly blend realistic virtual objects with the real world. In AR apps, the device’s camera is used to present a live, onscreen view of the physical world. Three-dimensional virtual objects are superimposed over this view, creating the illusion that they actually exist. The user can reorient their device to explore the objects from different angles and, if appropriate for the experience, interact with them using gestures and movement.” With their track record on iOS Apps for iPhones and iPads, that may give them an edge.

Not to be overlooked, however, is the now famous and impressive Oculus Rift virtual reality headset, now a Facebook company. It’s still expensive but very competitive. “Content for the Rift is developed using the Oculus PC SDK, a free proprietary SDK available for Microsoft Windows (OSX and Linux support is planned for the future).[62] This is a feature complete SDK which handles for the developer the various aspects of making virtual reality content, such as the optical distortion and advanced rendering techniques”, according to Wikipedia.

With Augmented Reality (AR), one can immerse themselves in games, education, work, movies or whatever. The big question, however, is will this be just a short-term fad or will it be enduring like the personal computer or smartphone? We just don’t know yet but the question of industry-wide standards versus proprietary platforms will likely be an issue. We do know that familiar tech giants are betting on VR and AR for current and future products. What is certain is that thin-film technologies are essential for manufacturing the LCD, OLED or other screens used in the requisite headsets. If you thought that building 4K or 8K HDTVs was challenging, VR and ER in small form factors will make it interesting. The VR headset options abound.

A Crowded Market

“By 2016 there were at least 230 companies developing VR-related products. Facebook has 400 employees focused on VR development; Google, Apple, Amazon, Microsoft, Sony and Samsung all had dedicated AR and VR groups. Dynamic binaural audio was common to most headsets released that year. However, haptic interfaces were not well developed, and most hardware packages incorporated button-operated handsets for touch-based interactivity. Visually, displays were still of a low-enough resolution and frame-rate that images were still identifiable as virtual. On April 5, 2016, HTC shipped its first units of the HTC VIVE SteamVR headset. This marked the first major commercial release of sensor-based tracking, allowing for free movement of users within a defined space. “ per Wikipedia [https://en.wikipedia.org/wiki/Virtual_reality_headset].

Is AI Ready?

“We are at an inflection point in the development and application of AI technologies,” according to the Partnership on AI [https://www.partnershiponai.org/introduction/]. “The upswing in AI competencies, fueled by data, computation, and advances in algorithms for machine learning, perception, planning, and natural language, promise great value to people and society.

“However, with successes come new concerns and challenges based on the effects of those technologies on people’s lives. These concerns include the safety and trustworthiness of AI technologies, the fairness and transparency of systems, and the intentional as well as inadvertent influences of AI on people and society.

“On another front, while AI promises new capabilities and efficiencies, the advent of these new technologies has raised understandable questions about potential disruptions to the nature and distribution of jobs. While there is broad agreement that AI advances are poised to generate great wealth, it remains uncertain how that wealth will be shared broadly. We do, however, also believe that there will be great opportunities to harness AI methods to solve important societal challenges.

“We designed the Partnership on AI, in part, so that we can invest more attention and effort on harnessing AI to contribute to solutions for some of humanity’s most challenging problems, including making advances in health and wellbeing, transportation, education, and the sciences.”

Some founding members of this Partnership on AI include Apple Inc., Amazon, DeepMind, Facebook, Google (Android, Chrome), IBM (Watson computer), Intel Corp., Microsoft, Sony (PlayStation) and the The Association for the Advancement of Artificial Intelligence (AAAI).

What Is Needed

In future blogs, we’ll revisit some specific AI and VR applications of interest and challenges.