Buried in a sea of Apple patent applications that were published earlier this week by the US Patent & Trademark Office, an Apple invention for an alternative method for strengthening sapphire glass came to light. On September seventeenth I posted a report titled "LG Wins Contract for AMOLED Apple Watch Displays & Apple Fulfills Patent," which showed that the new Apple Watch was using a new patent pending sapphire glass strengthening process that we first covered in a report in August using ion steaming. In Apple's latest patent pending invention, we see that a secondary engineering team has worked on an alternative sapphire glass strengthening process. Whether their new process will work itself across the entire Apple Watch family over time or is being aimed for future cover glass applications for the iPhone and iPad is unknown at this time.
Apple's Patent Background
Apple tells us in their patent background that corundum is a crystalline form of aluminum oxide and is found in various different colors, most of which are generally referred to as sapphire. Sapphire is a hard and strong material with a hardness of 9.0 on the Mohs scale, and, as such, is capable of scratching nearly all other minerals. Due to its brittle nature, it is susceptible to dramatic strength reductions as a result of small defects caused by interactions with its environment. Therefore, while a sapphire part may be very reliable leaving the factory, after time and use it may become less reliable due to an accumulation of minor damage.
Additionally, producing defect free parts can be very challenging. Brittle materials' strength are limited by the flaw population on the surface. An inconsistent or inadequate surface finishing can lead to very weak parts. Glass is chemically strengthened to a significant depth to minimize the effect of these flaws, but on extremely hard materials such as sapphire, a similar process is not readily available. For example, the sapphire's hardness makes cutting and polishing the material both difficult and time consuming when conventional processing techniques are implemented. Further, conventional processing tools such as cutters experience relatively rapid wear when used on sapphire. This further increases the resource demand when surface finishing sapphire parts.
Additionally, the use of parts formed from different materials, such as sapphire and glass, glass and plastic, sapphire and plastic, and so on, in devices can lead to differences in the optical appearance of side by side components due to the high reflectance of sapphire. Between two materials having particularly strong differences in reflectance the effect can be very noticeable.
Apple's Invention Relates to Surface Coatings on Cover Glass Such as Sapphire
Apple's invention generally relates to surface coatings on substrates. In particular, the patent application relates to coatings which strengthen the underlying substrate for use as a window on an electronic device, such as mobile phones and computing devices.
Apple later expands on the devices that their new invention could apply to, which includes: "tablet computer, notebook computer, instrument window, appliance screen and the like. Additionally, the device may include non-electronic devices such as mechanical watches which utilize a similar hard window."
Apple's invention covers systems for strengthening a sapphire which encompass a substrate having a first surface and a second surface with a plurality of surface defects.
The material (e.g. alumina) in the first layer may have a first hardness as measured in Mohs. Likewise, the material (e.g. alumina) in the additional layer may have a second hardness as measured in Mohs. The first hardness and the second hardness may be different. For example, the second layer may be harder than the first layer.
In accordance with various embodiments, the additional layer comprises a variety of materials. For example, the additional layer may be a polymer matrix having hard blocks suspended within the polymer matrix. The hard blocks may be configured to deflect into the depth and/or laterally within the polymer matrix. The polymer matrix and the hard blocks may have refractive indexes that match. In another example, the additional layer may be an anti-reflective coating having a refractive index less than the first layer.
In accordance with various embodiments, the sapphire substrate surfaces may be preconditioned. For example, at least the first surface or second surface may be an amorphous surface caused by ion implantation converting an original crystalline surface on the sapphire substrate into the amorphous surface.
Apple's patent FIG. 1a above illustrates an iPhone which may utilize a hard substrate as a window. A "hard" substrate is one that is generally scratch resistant, or more particularly one that has been treated to be more scratch resistant than the same material in an untreated form. For example, sapphire and alumina may be the same chemical composition of Al2O3. However, based on structural differences, alumina may have a slightly reduced hardness compared to sapphire's Mohs hardness of 9.
The iPhone may include cover #102 with a bezel #104 about all or a part of its outer periphery (e.g. edge) where bezel 104 is couplable to housing #106 in a manner that secures the cover to the iPhone (or other device). Depending on the application, the bezel and the housing may be formed of a variety of different materials including, but not limited to, polymer materials (e.g. plastics), metals (e.g. aluminum, steel, etc.), amorphous glass materials (= liquid metal), composite materials, and any combination of the noted materials.
The cover may include a variety of components for viewing the display of the iPhone (and/or other device). The particularly cover noted on the iPhone may have window #110 which may be a sapphire substrate provided to protect the components of cover 102 from damage.
In accordance with various embodiments, the window #110 may be manufactured from any of a variety of different materials. The materials may include transparent polymer, amorphous glass, and/or transparent crystalline materials.
The window generally may be formed from any of a number of extremely hard translucent materials. For example, extremely hard translucent materials may include sapphire. While sapphire's inherent strength is higher than that of amorphous glass (liquid metal), there is no well-established process to provide significant strength improvements after mechanical shaping, like chemical strengthening imparts to amorphous glass. Failures in sapphire are typically driven by propagations of surface defects under stress.
Consistent heavy use of a device such an iPhone having a sapphire protective substrate may cause defects in the surface. Even in the absence of environmental damage to the substrate, the substrate is unlikely to be manufactured flawlessly so surface defects are likely to be present.
In accordance with various embodiments, a coating may be applied to the surface to reduce the effects of the surface defects. The application of the coating may fill the surface defects. The application of the coating may form a beneficial layer on the surface of the sapphire substrate that may also provide a uniform outer surface that is substantially without defects. Providing the uniform surface may allow the substrate to endure higher stresses during use. While a first coating may address surface defects, a plurality of coatings may provide additional benefits.
As discussed above, a sapphire substrate #200 may have defects 202 and 204 on the surface as noted above in patent FIG. 2. While it may be counter-intuitive to apply a coating of a softer but similar material (e.g. alumina) to the sapphire, such a coating may strengthen the sapphire substrate.
For example, as the sapphire substrate is stressed in tension the defects are pulled apart which may cause failure. However, a coating may limit the amount the defects 202 and 204 are pulled apart under tension and may be beneficial as it may ultimately limit additional cracking of the substrate.
Apple notes that the coating may form a layer suitable to control distortions of the surface that could otherwise cause the defects (e.g. defects 202/204) to expand and cause failure of the sapphire substrate. By sealing the substrate surface defects the weakest points on the surface are reduced and/or eliminated. Sealing the defects helps prevent force from directly impacting the defects and causing additional damage. Creating a uniform layer puts a barrier between destructive external force and the stress risers in the defects.
The barrier may have the effect of spreading out the force of an impact and limit the localized stress at the defect. Various embodiments may employ silicon oxynitride (SiON) as a coating, while others may use alumina, and still others may use a combination of the two.
In accordance with various embodiments, the coating may be applied as a sacrificial coating. That is, the layer formed by the coating may be a sacrificial surface. Particularly, a softer surface (e.g. a surface of a material that is lower on the Mohs scale) may not suffer the damage that a hard surface (e.g. a surface of a material that is higher on the Mohs scale like sapphire) may suffer (e.g. damage penetrates to a shallower depth).
A less brittle surface may not suffer the damage that a more brittle surface may suffer (e.g. damage penetrates to a shallower depth). Less damage in the coating layer results in fewer defects capable of expanding into failure of the bulk substrate. In response to potentially damaging contact with the surface, the protective layer may absorb the damage and therefore allowing the substrate to maintain its high strength. In accordance with various embodiments, SiON and/or alumina may be a useful material for forming a protective layer.
Additionally, SiON and/or alumina applied to the sapphire substrate surface may have internal compressive stresses strengthening the alumina layer and applying a compressive force to the sapphire surface limiting distortion and failure of the defects. The internal compressive stresses may be applied to the alumina layer through the adhesion and/or deposition process.
During the coating process the sapphire substrate surface may be bombarded with ions to adhere the SiON or alumina to the sapphire substrate #200. Changing the coating parameters, such as the ratio of bombarding ions to SiON or alumina, may provide a layer with an internally high compressive stress.
Also, internal compressive stresses may be applied through the difference in coefficients of thermal expansion between the SiON or alumina layer and the sapphire substrate. For example, the SiON or alumina and the sapphire substrate may be heated prior to deposition of the SiON/alumina. After depositing the material, the sapphire substrate and the alumina or SiON may cool together. The differences in thermal expansion of the sapphire substrate compared to the coating material may cause the coating material to compress the surface of the sapphire, resulting in a sapphire surface layer with significant compressive stresses. Depending on the composition of the coating material layer, and the process conditions, the opposite effect could be achieved to yield a highly stressed SiON and/or alumina coating.
Additionally, applying a sufficiently high heat to the coating material may cause recrystallization of the coating material, whether SiON, alumina, or a combination thereof. Generally, these coating materials may be applied to a sapphire substrate as a film; the film is typically amorphous when deposited on the substrate. Heating the film may anneal the film, thereby causing formation of polycrystals from the amorphous film. By recrystallizing the coating material in this manner, the overall strength of the coating and resistance to scratches, cracking, and impact-related failures may be increased.
The coating layer may be applied with suitable adhesion via a number of different technologies. For example, the technologies may include plasma enhanced chemical vapor deposition (PECVD), ion beam assisted deposition (IBAD), physical vapor deposition (PVD), and/or chemical vapor deposition (CVD), each producing a slightly different structure to the layer.
The different structure may affect the hardness, strength, and/or optical properties of the final part. The deposition of the coating materials varies by process, with the specific conditions, including the atmosphere, the temperature of the substrate and chamber, the pressure, presence, ratio, type and energy of additional energetic ions, the deposition rate and the condition of the applied coating material, all contributing to the final structure, composition and density that can affect the various material properties.
A Polymer Matrix
As illustrated in Apple's patent FIG. 6a above, an exterior layer 600 may take the form of a polymer matrix #620 having a plurality of hard blocks #610 suspended within the polymer matrix. In some embodiments, the plurality of hard blocks may form a single row at the surface of the matrix.
The effect of this structure is that, when applied as a surface, the exterior layer may provide the strength, feel and/or hardness properties of the hard blocks but have the flexibility of the polymer matrix. Providing a number of rows of the hard blocks in the polymer matrix may create a layer that possess sufficient rigidity (and other mechanical properties) to be utilized as a substrate in many applications, such as for use as a replacement for a cover glass in an electronic computing device.
Apple's patent FIG. 7 illustrates a schematic view of surface processing apparatus for applying a surface treatment to a substrate.
A uniform coating may be applied to both the sapphire and plastic portions of the mobile device. The coating or film may be chosen to ensure both parts, once the film is applied, have a relatively uniform specular and/or diffuse reflection, thereby optically matching this aspect of the materials to one another.
A second substrate may also be applied to the device. The second substrate may be any part of the device such as a button, bezel, cover, or similar part (see, e.g., FIG. la). The second substrate may be formed from any material. For example, the material may be glass, plastic, aluminum, carbon fiber, or any other material that may be used on a device.
Apple's patent was originally filed on March 14, 2014 and published this week by the US Patent and Trademark Office. Considering that this is a patent application, the timing of such a product to market is unknown at this time.
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