January 21, 2022
OLED Microdisplays are applied in interchangeable lens mirrorless cameras viewfinders (electronic viewfinder: EVF), head-mounted displays (HMD), etc. We have developed an OLED Microdisplay with unparalleled resolution in 2021, ten years since its first launch. With its outstanding quality and high definition, this display offers great potentials for applications in wearable devices with AR (augmented reality) and VR (virtual reality) capabilities—such as ones we often see in near-futuristic movies.
The context in which this small 0.64-type display with its compelling high image quality became a reality, is a story of relentless efforts and solidarity of Sony Semiconductor Solutions Group (hereafter “the Group”) development team members.
Tamura：In simple terms, it is a display device comprising of a light-emitting diode formed by organic electroluminescent (EL) layer on top of the pixel arrays and peripheral circuits constructed on a silicon backplane substrate(circuit board). They are mainly used in EVF of interchangeable lens mirrorless cameras, AR/VR eyewear, and HMD. Our products are unique in terms of their small sizes and high definition. We mainly develop devices of 0.2 type to 0.7 type. These OLED Microdisplay products offer great potentials for AR/VR applications in glasses, goggles, HMD, as well as scope of applications, and are expected to expand further in the future.
Kon：Normally, the backplane is made of glass for devices such as PCs and smartphones, whereas we use silicon backplanes in our products. Silicon facilitates to significantly improve the display’s image definition.
Kato：It has been 10 years since Sony made the first batch of shipment of OLED Microdisplays. Initially, we were developing ultra-small displays as we already had expertise in silicon backplane technology and organic EL frontplane technology, and this was the beginning of the development of our OLED Microdisplays.
Kato：The biggest advantage is the omission of the backlight parts, which is generally required for LCD. You can see the difference in an electronics retail store, for example, by comparing LCD and OLED screen TVs. The color black is deeper on the OLED because of their own light emission ability, it does not have the problem of light leakage from a backlight.
Tamura：It was originally developed for EVF of Sony’s cameras. When interchangeable lens mirrorless cameras came into the market, we needed to develop an EVF to replace the conventional optical viewfinder due to the device structure of the camera. Then, we aimed to create a better version with a high-definition capability, which led to the development of the OLED Microdisplay. Subsequently, we continued improving the device, enriching the lineup with various sizes and definition variations. The product range grew, and today, these are mounted in the devices of our external clients, too.
Kato：The early models were not exactly super high definition, as the pixels were quite discernible on the display. They also had some problems in terms of the viewing angle properties, causing cross-color when viewed from off angles. We particularly worked hard on reducing the pixel pitch to attain higher image definitions and improve the viewing angle performance.
Kon：The development this time had several challenges to overcome. The product size was something we tried for the first time. It was by client requests that they needed larger EVF.
Kato：While there is demand for smaller OLED Microdisplays as they are mounted on cameras, there is a limit to how small they can be in order to ensure visibility. Also, even if the display could be magnified through a lens, images would be pixelated and affect the visibility if the display has low resolution. So, we considered the minimum possible pixel pitch realizable using the process and device technologies we had.
Tanaka：I worked on the wafer process design, and it involves the design of component layout on a wafer. Aiming to achieve smaller pixels to be mounted on a larger display, I pursued the development of a micropixel by reducing the size of the existing 7.8µm pixel down to 6.3µm. To achieve this, I worked on reducing the semiconductor circuit wiring pitches, and starting design validation to improve the capacitors performance.
Tamura：As far as I was concerned, I engaged the camera design division early on in the specification design phase because the product was intended for Sony’s cameras, and asked them to collaborate throughout, from prototype evaluation and feedback to the mass production phase. The unprecedented large size of the device raised doubts among some members of the development team as to whether such a large device would really have suitable applications. I took the time to discuss and explain the client requests so that everyone was comfortable with the project.
Kon：I was responsible for peripheral circuits, and the first thing I did was to brainstorm with the team and consider solutions for each challenge suggested. Then, we verified the feasibility of these ideas through simulations and so on. The ideas were then reflected in the actual designs and layouts, and we pursued prototyping and proof of concept in order to verify the implementation and operation of manufacturing processes.
Kato：When the pixel pitch is reduced to achieve higher definition, the viewing angle properties are considerably compromised and result in undesirable color mixing when viewed from off angles. In order to improve this, the luminescent parts must be closer to the CF (color filter). Conventional OLED Microdisplays have the CF directly on the glass substrate encapsulating the organic EL layers. This arrangement increases the distance between the light source and CF, also limits the arrangement accuracy during glass attachment process. To overcome these difficulties, we decided to introduce a process for the on-chip color filter*2 (OCCF). This technique allows placement of the CF closer to the OLED, which was the light source, and significantly improved the viewing angle properties. In addition, it allows an incorporation of Sony’s unique semiconductor processes, which helped to further improve the accuracy of the positioning in relation to the light source.
*2: Technology to separate color spectra from the white light emitted from the OLED.
Moreover, we designed a new pixel layout because simply laying the CF directly on top of the OLED resulted in compromising the viewing angle properties, causing undesirable color mixing when viewed horizontally as well as perpendicularly. As we continued fine tuning until the last minute of the development, we managed to make a significant improvement in this and realized remarkable performance against the competitor.
Kon：I believe that our devices are superior to our competitors by far. I feel, our attention to details is truly remarkable.
Tamura：That is true. When the prototype product is completed, I take it to our clients. They will put it against a lens and check it by looking at the image from various angles. If they detect even the slightest cross color effect, they will give the prototype product back to us saying they are not sure about it. So, we do need to do our very best to achieve the ultimate viewing angle properties.
Kato：The commercialization team is also involved to the very end of the project. Even in the last minutes of finalization, and I had many discussions with Kon-san and Tanaka-san about how it could be further improved, based on the data throughout the development phases.
Tanaka：In the wafer process design, where I was involved, it was mandatory to achieve high-capacity elements and smaller wiring pitches in order to enable higher-definition pixels. As the conventional processes were unable to realize this, it was necessary to create device structures and design rules from scratch.
As for the reduction of the wiring pitch, I had discussions over and over again with device and circuit designers to find target design of the narrowest pitch and the lowest possible resistance that would avoid short circuits, which became the basis of our device structure designs.
Meanwhile, a micropixel means a smaller area available for the capacitor. This necessitated to reduce the thickness of the dielectric layer to ensure the same level of capacity as conventional models. We experimented varying the layer thickness and tried different processing methods many times to identify the maximum reduction of the thickness while accommodating the device characteristics properties and process fluctuations.
To put it simply, my task was to reduce whatever was possible, from the wiring pitch to the film thickness, in order to meet the requirement “to make everything that was in a bigger box to fit in a smaller box.” It was a challenge to realize the ultimate modification of the process in this generation.
Kato：The development started with the mass production phase in mind from the early stages. Our development was carried on through analyzing thoroughly to what extent we could go before it started affecting yields through mass production.
Tanaka：This product has been stable at high levels of yields since the production started. I believe this is owed to the meticulous planning and designing early in the development stages.
Tamura：Apart from EVF, OLED Microdisplays are being applied in AR/VR glasses and medical HMD.
In the EVF application, it offers a significantly higher image quality compared to conventional LCD in terms of the contrast and color reproducibility at low levels of luminance, which is highly appreciated by many clients. Its small sizes and low power consumption also make the application to mobile and wearable devices easy, and we expect that the scope of application will expand, including the AR/VR devices.
Kon：As I mentioned at the beginning, we can achieve very high definition in the display by using a silicon backplane. Kato-san, Tanaka-san and their team have been pursuing higher resolutions in their efforts to realize smaller pixels. We will continue our efforts in making further improvements.
There is an index called ppd (pixel per degree), which measures the perceived visual quality of images. The higher the value is, the closer the display image is to the real vision. As you look at a low-definition image, you may notice the pixel dots in it. As we continue to improve the image definition, I believe it is possible to reach the level where the display image is no less in quality than viewing the actual object.
I suppose that the EVF mounted in the Sony’s flagship interchangeable lens mirrorless camera, α1, that offers high quality image close to real-life image.
Kato：I think people normally use the EVF in their cameras only when they are capturing images, but they can also use it to verify the captured photos and videos. I think you will be surprised to see how clear the images are compared to viewing them on an LCD display camera.
Kon：Other than these, it could be applied in eyewear with supporting features for people with impaired color vision or low vision. This is still in its early days, but there are certainly many potential areas where it could be leveraged.
Kato：So far, they have mainly been intended for EVF, but they are increasingly adopted in the AR and VR device applications today. However, the AR/VR devices are relatively large, which makes application cumbersome. In VR devices, immersive experience is very important, and for this reason, there is demand for even larger displays. Our product this time is a 0.64 type, but clients in the VR business request larger displays than this.
Meanwhile with AR device applications, luminance poses a challenge. AR devices involve see-through components, which let the ambient light pass through. The luminous energy of ambient light is immensely high that it overwhelms the OLED Microdisplay of today’s specs. As a result, images on the display become unrecognizable. So, the key is to improve the display luminance to close the gap.
Kon：Our OLED Microdisplays have sufficient luminance as they are today, but the complex optical design in AR devices consumes the luminous energy of the display. Therefore, we need to further increase the luminance. Once high luminance is achieved in a display as thin as a spectacle lens, there will be many different contexts to which the display can be applied.
Tamura：As long as I am with the Group, I would like to take on any challenging projects. We have so many products that other companies don’t, and it would be wonderful to be involved in developing such products.
Kon：I would love to be working on interesting devices, even beyond OLED Microdisplays. I think devices are like a treasure trove of cutting-edge technology. As it normally takes several years to introduced in a product and its launched, it is always exciting to witness the latest view before it becomes available in the market. As the the Group is developing many products, apart from the OLED Microdisplay, which would be the first in the world, there are many choices that appeal to me to get involved.
Kato：I expect that the market will grow for small size AR and VR devices with OLED Microdisplays. It is still not so big today because products are not aligned with the needs. Smaller devices with excellent immersiveness and visibility will be in great demand. The needs are clear in the line of AR and VR. So, I would be interested in creating more advanced super-microdisplays and contributing to the development of AR/VR devices.
Tanaka：With the product we developed this time, I explored the ultimate possibility of miniaturizing pixels. I think there is still some miles to go before we hit the level of image definition that truthfully reproduce the real views. So, I would like to continue with this challenge.
Another point is that the strength of our OLED Microdisplay is undoubtedly the high performance, but its high quality is equally a selling point.
In order to secure high quality and high yield in mass production, it is important to consider mass production in early developmental stages, build robust processes, and also establish the design rule in parallel to ensure robust design as they are developed, just as we have done this time. We have everything in place, from product design and development to mass production. I think this is precisely why we are capable of offering high performance and high quality in our products. I am currently transferred on a development division, and I would like to continue developing robust processes for the next generation processes by use of my experience of and expertise in manufacturing division.
Tamura：What Kon-san and Tanaka-san do is not fully appreciated by outsiders, but it takes a long time to develop products of this nature. It is therefore very important that they have the competence to realize devices of an acceptable level of completion from scratch. If they were spending much time in trials and errors, our competitors might beat us, or the product might be already obsolete when launched in the market. We can take our products to the market timely, and this is largely owing to the incredible efforts which our members of the development and process teams are making. Our strengths come, for sure, from the fact that we have in-house capabilities from the development to the manufacturing phases. But I think that the source of our strengths to produce extraordinary products is also found in the environment where everybody comes together and cooperates.