Re: Why is gold better than aluminum to make a 30% reflective layer

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 
mwnet@swbell.net. 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