Hello, my name is Eiji Ohno of the AV Core Technology Development Center. I'm in charge of developing optical disc technologies.

Here, I'm going to present some technologies for the single-side, dual- layer, rewritable Blu-ray Disc.
The achievement of these technologies was a truly epoch-making event in the more than 30-year history of optical disc development.

Eiji Ohno Manager Media Process Development Section AV Core Technology Development Center

This chapter introduces the Blu-ray Disc categories. There are three main types of discs, as described below.

BD-ROM/Playback Type（ROM:Read-Only-Memory）
This type of disc comes pre-recorded with movies, game software, etc.

BD-R/Write-Once-Only Type（R: Recordable)
This type can be written to only once. It is ideal for recording data that is not to be altered, or for use in easily distributing data.

BD-RE/Rewritable Type（RE: Rewritable）
This type of disc lets you rewrite the data about 1,000 times, making it similar to a DVD-RW disc.

Each type is further classified according to capacity, that is, single-side, single-layer (25-GB) disc, or single-side, dual-layer (50-GB) disc.

The term "single-side" means that there is a recording surface on only one side of the disc, with a cover layer that is 0.1-mm thick.

The laser spot in the Blu-ray Disc standard is much smaller than that in the DVD or CD standard because it combines a blue laser beam with a large NA objective lens. Theoretically, a small spot will tend to distort when the disc warps. Optical calculations have proven that a thinner cover layer is effective for preventing this distortion. After giving full consideration to this point, we chose a 0.1-mm cover layer to ensure reliability.

There are either one or two recording layers (films) inside this 0.1-mm cover layer. The number of recording layers determines the capacity. "Single-side, dual-layer/50 GB" means that there are two 25-GB recording layers inside the 0.1-mm cover layer. This gives you an idea of the incredible recording capacity that exists in this micron-sized world.

The Cross-Section of a Blu-ray Disc

When the BD standard was first established, it was decided that the Blu-ray Disc would use a single-side recording surface. The DVD standard uses discs with recording surfaces on both sides (called double-side discs). However, double-side discs have some problems, like having to turn the disc over after reading one side. It's also not practical from a cost perspective to make extremely thin, 0.1-mm cover layers on both sides.

Because of this, the technology for stacking the recording layers became vital as a way to increase the capacity of the Blu-ray Disc. We focused on how to put two recording layers into the 0.1-mm cover layer.

First, we decided to work on developing a dual-side rewritabledisc, which was considered to be the most difficult to produce from a technical viewpoint. It seemed that achieving the most difficult dual-layer BD-RE disc would also allow us to achieve dual-layer BD-ROM and BD-R discs.

Now, I'd like to talk about some Panasonic breakthroughs in the manufacturing method for single-side, dual-layer BD-RE discs.

The structures of single-side, single-layer and single-side, dual-layer discs are shown below.

In the single-side, single-layer disc, there's only one recording layer near the substrate. However, the single-side, dual-layer disc has an L0 recording layer, a space layer, and an L1 recording layer in the same 0.1-mm thickness.

The difficulty in achieving a single-side, dual-layer disc was to make a space layer of good quality using a low-cost material.

The purpose of the space layer is to separate the L0 and L1 recording layers to prevent mutual interference between their signals, to allow the laser beam to pass through to the L0 recording layer (which is behind the space layer itself), and to accurately return the laser beam to the pick-up head after it is reflected by the L0 recording layer. That's why extreme accuracy was required for the space layer, which has an average thickness of 25 µm (micrometers) and a thickness variation of less than ±2 µm. We also had to achieve this with a low-cost material and process.

A proposal to use a ready-made 25 µm film as the space layer was considered as one solution, but the cost of the equipment to produce the this kind of ready-made film and its transportation greatly increased the cost per disc. So we discussed the idea of fabricating our own space layer film using a low-cost raw material. As a result, we decided to use a UV curable resin for the space layer and began making test samples.

It was difficult to keep the thickness variation below ±2 µm, because UV curable resin is a liquid and its thickness easily varies. But the difficulty of the problem just made it more challenging to us. So we kept on working until we reached an idea that we call a "spin coating method." It uniformly coats UV curable resin with a 25-µm thickness over the entire 12-cm disc. This finally allowed us to stably produce a space layer with a thickness variation below ±2 µm.

We use the spin coating method for both the space layer and the transparent cover layer, so we can produce a large number of high-precision discs in a short time.

Another vital point was developing the materials for the recording layers.

We use phase-change recording technology to record and reproduce Blu-ray Disc signals. Panasonic has been actively involved in this technology since the initial development of DVD. It records data by using laser irradiation to change the atomic positions in each layer. Interestingly, under certain conditions a laser beam can return the recorded layer to its original, unrecorded form. Do you see the magic in this?

With rewritable BD-RE discs, phase-change recording repeatedly returns the disc to its original, blank condition, so new data can be written. This means that the BD-RE recording layer must be stronger than other types because it has to endure repeated laser irradiation. That's why the BD-RE disc was the most difficult to achieve among the BD-ROM, BD-R and BD-RE discs.

In order to successfully produce the recording layers, the L1 layer became the key to achieving "single-side, dual-layer BD-RE discs." When recording or reproducing  signals in the L0 layer, which is on the innermost part of the disc, the laser beam has to pass through the L1 layer. Because of this, the L1 layer must have sufficient permeability for the laser beam to reach the L0 layer and then for the reflected light to pass back through the L1 layer to reach the pick-up head. However, if the L1 layer is too thin, its properties as a recording layer decrease and this lowers its recording, reproducing and storage characteristics. So it was crucial that we simultaneously pursued both its permeability and its properties as a recording layer. While other companies were searching for ideas for these recording layers, we maintained our own, original research to find an ideal material for the L1 layer.

This finally led to our completion of the world's first single-side, dual-layer BD-RE disc. The final structure is shown below. The L0 and L1 recording layers actually consist of several films. Of course we're speaking on an atomic level here, which makes them virtually invisible.

Structure of the Single-Side, Dual-Layer BD-RE Disc

This was a difficult challenge even for skilled engineers like ourselves, and it gave us an excellent opportunity to put our accumulated experience to work.

Of course, producing only one perfect disc in the lab was not our goal. The most important thing for a manufacturing company is to produce a great number of high-quality goods at low cost and make them widely available. We conducted our development based on this concept. That is, we searched for materials that are high in quality and low in cost, and we looked for processes that would be applicable to mass production. This would give us the high-quality products we wanted at reasonable enough prices for the market. To do this, we found ourselves on the leading edge of the industry, both in basic materials research and in the development of mass production processes like our "spin coating method."

Our achievement of the single-side, dual-layer structure for rewritable BD-RE discs was a world's first. It was definitely a remarkable achievement in the history of optical discs.

We established the Blu-ray Disc standard with the view of eventually applying it to a multilayered structure.

Now that we've achieved the single-side, dual-layer structure, we are going ahead with a project to create single-side, four-layer discs with 100 GB of capacity.

Panasonic first began developing recordable optical discs in the 1970's, before the CD had even appeared. Our specialists created a pathway that led from the launch of the DVD standard to the current standardization and mass production of the Blu-ray Disc. By being able to foresee future trends, grasp the needs of our customers, and continue researching optical discs over the years, we were able to achieve a single-side, dual-layer BD-RE disc with a 50-GB capacity that left the competition behind.

[Related Link]

• Panasonic Advantages|BD-R/RE Discs