Sapphire is hard, strong, optically transparent and chemically inert.
BY WINTHROP E. BAYLIES and CHRISTOPHER JL MOORE, BayTech-Resor LLC, Maynard, MA
Have you ever wondered what blue gemstone earrings, an LED lightbulb and an Apple Watch have in common? The answer (at least for this article) is that all depend on sapphire as part of their manufacturing process. In part 1 of the following two part article, we will discuss how sapphire is becoming an important part of the mobile device food chain. Part 2 will concentrate on how sapphire is used in LED production.
Sapphire (chemical composition Al2O3) has a high melting point of 2040°C (3704°F) and is chemically resistant even at high temperatures. It is an anisotropic material meaning that its mechanical/thermal properties depend on the direction of the crystal plane that is cut and polished. An insulator with a 9.2 eV energy gap it is optically transparent. With a hardness of 9 on the Mhos scale, it is almost as hard and strong as diamond (10 Mhos).
To summarize, sapphire has some good points: hard, strong, optically transparent and chemically inert (there is a reason high end watches use sapphire crystals) and some bad points: hard, strong, and chemically inert (which is why sapphire crystals are more expensive than glass). That is, the very properties that make it ideal for applications needing mechanical strength and hardness mean that it is a difficult material to grow, machine and polish.
There are several places where sapphire can be (or is now) used in the manufacture of mobile devices. The most publicity in this area was generated in 2014 with significant speculation in both the trade magazines and newspapers (such as the Wall Street Journal) that the iPhone 6 would be released with a sapphire touch screen or at the very least a sapphire cover glass over the existing touchscreen. Part of this speculation was fueled by the large number (1700 to 2500 depending on source) of sapphire producing furnaces being installed at an Apple facility in Mesa Arizona. However, the sapphire iPhone 6 was not released due in part to the difficulties in growing and processing enough sapphire screens at a reasonable cost to supply the significant number of phones produced. There are now sapphire touch screen phones available from other suppliers and recently, the Apple Watch was released with a sapphire screen. In addition, many fingerprint sensors and camera cover glasses are now produced using sapphire as the cover material.
Requirements for sapphire material is clear (forgive the pun). For screens and cameras, it must be of good optical quality i.e. transmit light well and have low surface roughness. For fingerprint sensors, it needs consistent surface quality and electrical properties.
FIGURE 1 shows a schematic of the production process for sapphire used in a mobile device screen. The following paragraphs provide more detail on this process  as well as a few of problems encountered along the way.
The sapphire production process starts when a seed crystal and a mixture of aluminum oxide and crackle (un-crystallized sapphire material) is heated using a specific temperature/time profile, then cooled (this process can take two weeks depending on the amount of sapphire being produced) using a carefully controlled set of time/temperature profiles. When done correctly, the cookie sized seed grows and produces a single-crystal sapphire boule. That at least is the theory. In reality, two weeks is a long time and any number of problems can go wrong during this process including gas bubbles, mechanical faults such as cracks and contamination. Each of these problems can affect the sapphire and its optical/electrical properties. There is a clear correlation between the time taken to grow a boule and the potential quality of the boule produced. Many of the problems encountered in the upscaling of the sapphire production process sprang from trying to grow large boules at high speeds.
It is at the next step in the process where boule size does matter. Typically, the boule will be drilled or cut to produce material near the size needed for the particular application. It makes a significant difference if the material is for a watch crystal (say 1.5 inch diameter ~ 1.7 square inches). Here you can “core-drill” a boule to produce a number of smaller cylinders. For a phone screen/cover plate (at 4 by 6 inch i.e. 24 square inches) a larger portion of the boule is needed for a box shape. The ability to grow large sized boules on a regular basis is not in question; most important is how much of that boule is bubble-, crack- and impurity-free. In some cases the boules are inspected with various metrology techniques to determine which sections of the boule can be used and which cannot. The section of the boules not used is recycled into the original growth process (unless contaminated).
Given the hardness of the sapphire, diamond wire saws or diamond core drills are used for cutting or coring the boules. The yield from any boule is a function of the original boule size, the size of the cores or slabs being produced and the volume of the boule free from imperfections. As was discussed earlier, and is typical of many processes, the larger the size of the piece the lower the yield.
The next step is to take the cylindrical cores (or rectangular slabs) and cut them into appropriate sized pieces. The thickness of the desired part and the amount the producer is willing to invest in high technology solutions determines what is done next. On one end of the technology scale, the parts are cut using a wire saw or an abrasive cutoff saw. On the other end of the scale, you can ion implant the surface to produce a damaged layer at a depth below the surface determined by the original ion energy. If the slab is heated after sufficient implantation is done, a thin sheet will separate from the surface. Both processes result in parts of the approximate size needed for the application; a discussion of the pros and cons of each approach is beyond the scope of this article.
The process after this point depends on the parts’ final application and their manufacturer. Given the difficulty of polishing a material this hard many of the bigger companies have developed proprietary process for grinding or mechanically polishing the sapphire parts to the desired shape and surface roughness/finish. From a mechanical strength standpoint, it is important that there be no significant scratching of the surface or chipping of the edges which could severely limit the mechanical strength of the final piece. From an optical standpoint, it is important to produce a uniform finish so as not to effect the overall appearance of the part. At this stage, the parts are then ground to their final size and any additional shaping of the part including holes/ profiles is done. FIGURE 2 shows a variety of sapphire parts at this stage of the process.
In most sapphire part production these parts are next coated with a variety of optical and/or electrical and/ or chemical films again depending on their application. Because of its high index of refraction (1.76) a sapphire screen or watch crystal is highly reflective. For this application, the parts are typically coated with a series of films to produce an anti-reflection coating enhancing final screen readability. For parts that will be touched on a regular basis such as touchscreens or fingerprint sensors coatings, it is important that they be “self-cleaning.” In these cases, hydrophobic and oleophobic coatings are used to make sure your fingerprints are less likely to stay behind after the material has been touched. FIGURE 3 shows a series of parts after the coating and silk screening process. They are now ready for assembly into the mobile device.
The use of sapphire in mobile devices is driven by two main concerns. One is that the final screen/sensor be mechanically stronger and harder than most glasses. There are a number of videos  available showing cement blocks being dragged over cell phones to show the sapphire screens’ scratchproof capabilities. The second (and not as well known) factor is the significant data showing that touch sensors made using sapphire have better performance characteristics due to its superior electrical properties and electrical uniformity. This allows the development of sensors which have improved performance in the field.
The downside of using sapphire remains its cost. Estimates  have reported sapphire costs 2 to 10 times the price of an equivalent glass part. Although these costs are coming down, in price sensitive applications glass continues to dominate at this time and it is expected that only higher end phones will use sapphire screens.
In the second part of this article, we will discuss the importance of sapphire in the LED industry and the difference in process needed for this material.
Additional reading/viewing material
1. http://www.businessinsider.com/how-sapphire- glass-screens-are-made-2014-9
2. Video Aero Gear’s Flight Glass SX Sapphire Crystal vs a Concrete
3. http://seekingalpha.com/article/2230553-ignore- the-sapphire-threat-corning-is-on-a-roll