|MadSci Network: Physics|
Hi Michael! Good question! It seems it's actually possible to use Aluminum in the semi-reflective layer of a DVD. However, it hasn't been very practical from a manufacturing standpoint. A good semi-reflective layer (often referred to as "L0") material has the following qualities: · High reflectance (18-30%); · Good UV transparency; · Excellent stability; and · Good bonding characteristics. (Paraphrased from several of the links I list below.) Aluminum is more reflective than gold. So much more so, the semi-reflective layer has to be impracticably thin. Furthermore, Aluminum corrodes at high temperature and humidity levels. (Water vapor sneaks through the polycarbonate substrate and corrodes the pits, and any manufacturing defects found on a DVD. Laser light on corrosion can lead to information loss, which is exactly what you don't want to happen to a DVD.) Gold is not as easily corroded as Aluminum, and it is much more forgiving in terms of layer thickness. The one downside to Gold is that it doesn't bond as well. (There are adhesives that can help get around this, but they enhance the corrosion of Aluminum!) Still, Gold was far and away the "first choice" material for the semi-reflective layer. (Silicon works, but it's brittle, and subject to micro-cracking. This too can lead to information loss.) Other more promising materials have been developed. Various dielectrics seem to work well, as does combination of silver and other elements, often called "DVD Silver". Aluminum works well as part of this. Copper, blue lasers…there are a lot of things involved in making this work better and for less. Always more "in the works" with this developing and profitable technology. I really like your question because it looks at the history of the manufacturing, marketing, and "hype" surrounding an "evolutionary step" in optical data storage technology which developed, matured, and "moved on" in less than ten years! (Check out the dates for the articles in the links I list below. They tell the "real story". Amazing!) If you'd like to know more, please feel free to e-mail me at firstname.lastname@example.org. I've really enjoyed learning about this, and would be pleased to research further. Your MadSci, -Matt P.S. - If you want more detailed info, go to dogpile.com and search for "semi-reflective layer" and "sputtering". Links: DVD itself: http://www.steena.freeserve.co.uk/DVDindetail .htm An overview: http://www.d isctronics.co.uk/manuf/dvdmanuf/mfgdvd3.htm How do you make / code it?: http://members. tripod.com/therocket/dvd/howmade.html Gold in DVD's: http://www.kipin et.com/tdb/tdb_feb97/tdb-feat3.html Element-Al DVD: http://www.kipinet. com/tdb/tdb_may99/element.htm Layers upon Layers: http://www.kipinet.co m/tdb/tdb_aug97/feat2.htm DVD: How much can Gold hold?: http://www.cd-in fo.com/CDIC/Technology/DVD/dvd.html Format info with a mysterious dead Imation link at the bottom: http://www.neoucom .edu/Cruce/Dvd98/structure.html Layers and DVD Silver: http://www.kipinet.c om/tdb/tdb_jun99/layers.htm Copper, Silver?: http://www.targettechnology.com/ "The wars" in '97: http://www.ltx.com/ltxtoday/ june97.html Tools of the trade: http://www.nissei-md.com/ Metallizer: http://www.flt.com/tn.html A "monoline" machine: http://www.flt.com/unidvd.html The future? (See DVD-18): http://www.wamodvd.com/ Spinning Down?: http://www.emedialive.c om/EM1998/spin10.html Recycling!: http://www.kipinet .com/tdb/tdb_apr99/precious.htm
Try the links in the MadSci Library for more information on Physics.