There has been a lot of talk lately about the adoption of sapphire in mobile electronics since several manufactures have brought to market smartphones as well as smartwatches with sapphire displays. Clearly the interest in sapphire displays is growing, but why is the industry moving towards this material?
The first and biggest reason why manufacturers are shifting towards sapphire is the material’s innate hardness. Hardness affects a number of properties of materials, but one of the key properties affected is the ability to be scratched. In general, a material can scratch another material if it is harder and cannot scratch a material if it is softer. Most of today’s mobile displays use glass as the display material. Typically glass has a hardness of around 6 on the Mohs scale of hardness. This makes glass harder than many things, but still softer than quartz (7 on the Mohs scale), as well as some other materials. It turns out that being harder than quartz in pretty important. Quartz is what makes up sand and is also one of the primary materials used in cement. So there is a pretty good chance that your phone will come in contact with quartz at some point. Sapphire, on the other hand, has a hardness of 9 on the Mohs scale. There really isn’t much that can scratch sapphire, other than diamond. Before the widespread adoption of the mobile phone, there was only one display that most people carried around with them, a wristwatch. Whereas watches have used sapphire material for ages in order to prevent scratching or breakage, this is a new territory for the much larger displays used in modern smartphones.
While hardness is the most obvious and the most immediate reason for displays switching to sapphire, there are additional reasons. For one, sapphire is a crystalline material. Being crystalline means that every atom in the material is ordered. Glass, on the other hand, is amorphous, meaning that it has no regular atomic structure. Much of the microelectronic industry is based on exploiting the properties of crystalline materials, and a single-crystal substrate is generally required to grow a single crystal film. Sapphire, in addition to having a crystalline structure, is also highly resistant to extreme temperatures and harsh chemical environments that would cause glass material to break down. Because many of the thin films used in the electronics industry are produced in extremely harsh environments, glass is really not an ideal substrate for depositing these films. So, switching from glass to sapphire for displays offers up new possibilities for how the displays can be used.
One interesting potential application for sapphire displays is as a solar optic. While both glass and sapphire are transparent in the visible spectrum, sapphire is much more transmissive outside of the visible spectrum—and most light is outside the visible spectrum. Glass absorbs much of the UV and IR light that would pass through sapphire. As phones become thinner there is less and less space that can be occupied by a battery. As a result, phone manufacturers are forced to find new ways stretch the lifetime of the phone battery. One way to do this is to integrate solar receptors into the display. However, getting the most impact from the solar cells requires maximizing the amount of light that is transmitted, which makes sapphire very appealing.
Mobile devices create a unique opportunity for optical materials, and sapphire is being specified more and more frequently to fill this role. It has become commonplace for the camera optics to be made out of sapphire, and the number of phones with sapphire in the primary display are increasing. I have touched on some of the reasons why this change is needed, but there are more. For example, sapphire can aid in structural integrity of the device (re: bendgate), and can help with passive cooling, due to its increased thermal conductivitiy. For all of the reasons that sapphire is superior to glass, the change isn’t just inevitable, it is necessary. With this change to more advanced materials will come all sorts of unpredictable new advancements in the mobile electronics industry. Whereas glass is little more than an optical interface to the underlying display, sapphire will become part of the structure. Sapphire displays will drive new technical capabilities, and will do so without scratches or wear.
John Ciraldo has a degree in Engineering Physics from The Ohio State University as well as masters’ degrees in Physics and Material Science & Engineering from The University of California at Riverside. He is currently employed at a Technical Manager for Rubicon Technology where he oversees R&D projects primarily related to material synthesis for the mobile device market.