Galvanics: the critical link to perfect Blu-ray

 

By: Leslie Derrig

 

Galvanics has always been referred to as voodoo when in actuality it is a very controlled science. The precise discipline of depositing nickel ions onto a surface, within microns of uniformity, has been the objective for standard DVD plating, but with Blu-ray the parameters have tightened and plating specialists have sought answers to the various new issues that have resulted.

Every Blu-ray facility faces similar challenges and each looks to find answers, but each chooses to solve these issues behind closed doors. Who can blame them? This is a competitive business and the high degree of success in Blu-ray is the new target.

There are several Blu-ray mastering systems on the market, and each poses its own issues unique to their specific technology.

ODC-Nimbus offers a system which utilises standard glass substrates and a high definition version of its dye polymer. In this case, the galvanics process varies little from DVD, with the appropriate tightening of clean room protocol and process parameters.

The Sony PTM mastering process uses a 200mm x 0.7mm silicon wafer. Again, much of the galvanics process is essentially the same as for DVD in terms of chemistry, though some process parameters will be modified.

The Singulus Crystalline BD Mastering System uses PTM (Phase Transition Mastering) technology with standard 180mm glass. This process is slightly different as it produces a Mother from the master, instead of a Father.

M2’s SQM-based BD solution will create a Father from a glass substrate with a centre hub and will require similar Galvanics process steps as DVD. The glass processes in most of the Blu-ray mastering systems are generally capable of recycling for reuse.

 

Plating parameters

 

Because of the expense and time in cutting a Blu-ray wafer, it is imperative to produce a good Father or Mother off the master. When it comes to Blu-ray, most galvanics engineers do not concern themselves with fast stampers. Instead, the goal is to produce a high quality stamper that is worthy of the Blu-ray format. So speed is no longer the paramount issue – at least for the time being.

Plating times are varying from 65 to 80 minutes. Plating technicians are also finding that if a part is run too fast, it could jeopardise the complexity of the metal causing an unacceptable degree of stress.

 

Cleanliness

 

As in DVD, Blu-ray clean rooms are generally in a standard Class 10,000 facility. As a result, we do see the need for filter changes to occur more often: around once a week is typical. The DVD pit geometry is measured in microns whereas with Blu-ray it is measured in nanos.

Small micron-sized or large nano-sized contaminates can ruin a stamper, so it is more critical than ever to keep the bath filtered and to ensure that the surrounding area is free of dust particles and human dander. Standard clean room garb is essential and we are seeing a trend toward stricter clean room protocol. This represents a reversal of the trend toward more relaxed protocol, especially within the galvanics area. We saw a similar evolution when CD mastering facilities retooled for DVD, although the leap to Blu-ray is proving even more difficult.

Tighter controls on bath chemistry are also being implemented in order to ensure a successful transition to BD. This translates to titration tests at more frequent intervals. These tests make certain that bath concentrations are stable which in turn results in more stable yield.

Cleaning the anodes more frequently also assures purity in the plating bath. Anode bags are still used by some of the facilities, but not by all. Dummy plating at a low current is essential and performed more when the system is not being used.

 

Blu-ray workholders

 

When dealing with silicon wafers, special consideration must be given to their handling and fixturing. These wafers are fragile and can easily crack if the workholder is improperly designed, or not well maintained. All Blu-ray masters, whether on silicon or glass, need to be fixtured with the utmost precision in order to avoid stress and non-uniformity. The workholder therefore has become a major focal point once again.

Both Digital Matrix and SWELTech have designed cathode backplates or workholders using a vacuum chuck. The SWELTech holder is removed from the shaft to change parts and apply vacuum; the Digital Matrix system offers a vacuum line fed through the shaft allowing parts to be changed without removing the backplate from the shaft. With the Digital Matrix vacuum system, the contact rings are also held in place with the vacuum system before the outer ring is sucked into place, alleviating the pressure on the OD of the wafer.

Both systems remove the trapped air behind the substrate, allowing flatter Fathers, Mothers and Sons. BD stampers require thickness uniformity (TTV) of no more than ±3 micron deviation.

Digital Matrix has also designed a non-vacuum Blu-ray workholder that adapts to their standard system that is presently being used in many facilities with great success.

BD engineers have pointed out that it is imperative to have a process and workholder that is easy to use in production and that eliminates steps and over-handling. Old style workholders may work, but over-handling can cause damage to the master. Screwing or clamping unevenly can easily cause the tension to break the wafer.

Wafers are extremely pure and flat, allowing very flat metal off the wafer. The bigger hurdle seems to be in the production of flat metal from metal, better known as Family Making.

 

Attention to detail

 

Seasoned engineers mention some blu-ray mastering systems can get about 10 Fathers off the original substrate, while others can achieve up to 20. With the standard DVD plating process you could, for example, produce one Father, five Mothers and 25 Sons from one master. The economics of Blu-ray dictate that the numbers should be higher; the cost per master is much higher, so we would like to see more production from each master. This can be accomplished as long as new protocols and tighter process controls are implemented.

Many engineers would prefer not to give actual numbers, as they say the yield can fluctuate with different electroforming operators and other varying conditions. However, the goal of most mastering companies is to achieve 900 to 1000 good stampers off one master.

Contact Rings are one of the ‘beastly’ issues in Family Making. In the standard process, they can be used over and over again and are factored as one of the larger expenses in mastering, so reusing cuts the costs. However, with the BD process, producers are finding that they cannot reuse the rings as much, and that the separation becomes super critical. In this area many mastering engineers have asked that we do not share their knowledge or expertise due to the proprietary nature of their techniques. We can say, however, that passivation of the rings and extra care in separation are two key factors in this process.  Processes are being developed to eliminate the contact ring in family making.

DVD Stampers are generally plated to around 295 µms and Blu-ray tends to be a few microns more, around 300 µms. This also varies from mastering company to mastering company, Father to Mother and mould to mould.

 

Stamper finishing

 

Generally photoresist is not used in the Blu-ray mastering process, so the task of PR removal is not an issue. However, passivation is still necessary and more critical than ever for producing the Family Series. The electro-passivation process will vary depending upon the precise mastering system.

Passivation times, current, and solution temperature are all critical to successful Family Series. High temperatures are often the cause of a failed BD stamper. Extensive and pure DI water rinsing is also vital to a flawless stamper. A forced jet rinse and two-sided, spin-dry assures the removal of any leftover impurities.

Small contamination and defects have a much greater impact on Blu-ray error rates (SER or Symbol Error Rate) than with DVD PI Sum-8 rates. Handling and post-plating processing (finishing) require the white glove treatment. Mere nano dust can cause the rejection of a stamper.

Protective tape is still used by many companies, and some store the substrate in special sealed wafer containment cases. Both blue and clear protective tapes are being used for BD production. At least one facility uses a spun-on protective coating on the stamper, and has reported no problems with residue on the stamper.

 

Upgrade

 

Older plating systems can be used in the BD process with some minor changes. Upgrading workholders, baffles, and modified plating programs are generally required to make the leap. Following tighter process protocol will be, in large part, a key to success. Finally, a good dose of training and experience will go a long way toward producing good BD product.

Overall, the expense of upgrading the plating department for BD production is minimal in comparison to other facility changes, but the expertise and attention to detail by plating engineers is priceless. A perfect stamper for Blu-ray production is still the critical link to a perfect disc.

 

 

Leslie Derrig - Digital Matrix