Dokumentenidentifikation 
EP1573727 10.01.2008 
EPVeröffentlichungsnummer 
0001573727 
Titel 
DOPPELSCHICHTIGES OPTISCHES AUFZEICHNUNGSMEDIUM UND DER GEBRAUCH EINES SOLCHEN MEDIUMS 
Anmelder 
Koninklijke Philips Electronics N.V., Eindhoven, NL 
Erfinder 
MARTENS, Hubert C., NL5656 AA Eindhoven, NL; TIEKE, Benno, NL5656 AA Eindhoven, NL 
Vertreter 
derzeit kein Vertreter bestellt 
DEAktenzeichen 
60317827 
Vertragsstaaten 
AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI, FR, GB, GR, HU, IE, IT, LI, LU, MC, NL, PT, RO, SE, SI, SK, TR 
Sprache des Dokument 
EN 
EPAnmeldetag 
26.11.2003 
EPAktenzeichen 
037739059 
WOAnmeldetag 
26.11.2003 
PCTAktenzeichen 
PCT/IB03/05446 
WOVeröffentlichungsnummer 
2004055799 
WOVeröffentlichungsdatum 
01.07.2004 
EPOffenlegungsdatum 
14.09.2005 
EP date of grant 
28.11.2007 
Veröffentlichungstag im Patentblatt 
10.01.2008 
IPCHauptklasse 
G11B 7/24(2006.01)A, F, I, 20051017, B, H, EP

Beschreibung[en] 
The invention relates to a dualstack optical data storage
medium for recording using a focused radiation beam having a wavelength &lgr;
of about 655 nm and entering through an entrance face of the medium during recording,
comprising:
 at least one substrate with present on a side thereof:
 a first recording stack L_{0}, comprising a recordable type L_{0}
recording layer, said first recording stack L_{0} having an optical reflection
value R_{L0} and an optical absorption value A_{L0} at the wavelength
&lgr;, and the recordable type L_{0} recording layer has a complex refractive
index ñ_{L0&lgr;} = n_{L0&lgr;}  i*k_{L0&lgr;}
at the wavelength &lgr; and has a thickness d_{L0},
 a second recording stack L_{1} comprising a recordable type L_{1}
recording layer, said second recording stack L_{1} having an optical reflection
value R_{L1} and an optical absorption value A_{L1} at the wavelength
&lgr;, and the recordable type L_{1} recording layer has a complex refractive
index ñ_{L1&lgr;} = n_{L1&lgr;} i*k_{L1&lgr;}
at the wavelength &lgr; and has a thickness d_{L1}, said second recording
stack being present closer to the entrance face than the first recording stack,
 a transparent spacer layer sandwiched between the recording stacks, said transparent
spacer layer having a thickness substantially larger than the depth of focus of
the focused radiation beam.
An embodiment of an optical recording medium as described
in the opening paragraph is known from
Japanese Patent Application JP11066622
.
Regarding the market for optical recording, it is clear
that the most important and successful format so far is a writeonce format, Compact
Disk Recordable (CDR). Although the takeover in importance by Compact Disk ReWritable
(CDRW) has been predicted since a long time, the actual market size of CDR media
is still at least an order of magnitude larger than for CDRW. Furthermore the most
important parameter for drives is the maximum write speed for Rmedia, not for RW.
Of course, a possible shift of the market to CDRW is still possible, e.g. because
of Mount Rainier standardization for CDRW. However, the Rformat has been proven
very attractive due to its 100% compatibility with read only compact disk (CD).
Recently the Digital Versatile Disk (DVD) has gained market
share as a medium with a much higher data storage capacity than the CD. Presently,
this format is available in a read only (ROM) and a rewritable (RW) version. Next
to the DVD ReWritable (DVD+RW) standard a new recordable (R), i.e. write once, DVD+R
standard was developed. The new DVD+R standard gets increasing attention as an important
support for DVD+RW. A possible scenario is that the end customers have become so
familiar with an optical writeonce format that they might accept it more easily
than a rewritable format. Recently a new format has been introduced called Bluray
Disc (BD) with even a higher storage capacity. For this format also R and RW versions
will be introduced.
An issue for both the R and RW formats is the limited capacity
and therefore recording time because only singlestacked media are present. Note
that for DVDVideo, which is a ROM disk, dual layer media already have a considerable
market share. A duallayer, i.e. dualstack, DVD+RW disk is probably feasible. However,
it has become clear that a fully compatible disk, i.e. within the reflection and
modulation specification of the duallayer DVDROM, is very difficult to achieve
and requires at least a major breakthrough for the properties of the amorphous/crystalline
phasechange materials, which are used as recording layers in e.g. DVD+RW media.
Without a full compatibility, the success of a duallayer DVD+RW in the market is
questionable.
In order to obtain e.g. a dualstack DVD+R medium which
is compatible with the duallayer (=stack) DVDROM standard, the effective reflectivity
of both the upper L_{1} layer and the lower L_{0} layer should be
at least 18%. More generally it can be said that any new generation dual stack medium
requires a minimum effective optical reflection level R_{min} in order to
meet a specification, e.g. for a dual stack BD the expected value of R_{min}
is 0.04 and for a dual stack BD compatible with a single stack BD R_{min}
= 0.12. Effective optical reflection means that the reflection is measured as the
portion of effective light coming back from the medium when e.g. both stacks L_{0}
and L_{1} are present and focusing on L_{0} and L_{1} respectively.
The conditions, which must be imposed on the optical reflection, absorption and
transmission values of the stacks in order to meet such a specification are by far
not trivial. In
JP11066622
nothing is mentioned about requirements with respect to optical reflection,
absorption and transmission values of the stacks. It should be noted that in this
document the normally used convention of notation of L_{0} and L_{1},
in which notation L_{0} is the "closest" stack, i.e. closest to the radiation
beam entrance face, has been changed: L_{0} now is the deepest stack, as
seen from the radiation beam entrance face, and L_{1} is the stack closer
to the radiation beam entrance face.
In
European Patent Application EP 1143431 A2
an optical recording medium of the type as described in the opening paragraph
is disclosed with A_{L0} = 12 % and R_{L0} = 20%.
The document
WO 02/099796 A1
used for the twopart limitation of appended claim 1 describes a dual
stack rewriteable DVD.
It is an object of the invention to provide an optical
data storage medium of the type mentioned in the opening paragraph which has an
effective optical reflection level of both the L_{0} stack and the L_{1}
of more than a specified value R_{min}.
This object has been achieved in accordance with the invention
by an optical storage medium, which is characterized in that A_{L1} ≤
1  R_{min}  √(R_{min}/R_{L0}) in which formula
R_{min} = 0.18 and in that the following formulas are fulfilled:
$${\mathrm{k}}_{\mathrm{L}\mathrm{0}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},\mathrm{+},\mathrm{\surd},\left({\mathrm{R}}_{\mathrm{min}}\right)\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{0}}\right)$$
and
k_{L1&lgr;}≤ {&lgr;*ln[1/(R_{min} + √(R_{min}))]}/(4&pgr;*
d_{L1}). For a given optical data storage medium, the effective reflection
of both recording stacks of a dualstack disc, should always lie above a specified
minimum reflection R_{min}. This implies that the effective reflection of
L_{1} should meet the following criterion:
$${\mathrm{T}}_{\mathrm{L}\mathrm{1}\mathrm{eff}}={\mathrm{R}}_{\mathrm{L}\mathrm{1}}\ge {\mathrm{R}}_{\mathrm{min}}$$
For L_{0}, the effective reflection should be
$${\mathrm{T}}_{\mathrm{L}\mathrm{0}\mathrm{eff}}={\mathrm{R}}_{\mathrm{L}\mathrm{0}}*{{\mathrm{T}}_{\mathrm{L}\mathrm{1}}}^{2}\ge {\mathrm{R}}_{\mathrm{min}}$$
Thus, we obtain a requirement for the transmission of L_{1} of
$${\mathrm{T}}_{\mathrm{L}\mathrm{1}}\mathrm{\ge}\mathrm{\surd}\left({\mathrm{R}}_{\mathrm{min}},\mathrm{/},{\mathrm{R}}_{\mathrm{L}\mathrm{0}}\right)$$
Equations (1) and (3) show that the optical properties of the total dualstack
medium are mainly defined by the optical properties of L_{1}. The combination
of equations (1) and (3) directly defines a requirement for the allowable absorption
of L_{1}:
$${\mathrm{A}}_{\mathrm{L}\mathrm{1}}\le 1{\mathrm{R}}_{\mathrm{min}}\mathrm{}\mathrm{\surd}\left({\mathrm{R}}_{\mathrm{min}},/,{\mathrm{R}}_{\mathrm{L}\mathrm{0}}\right)$$
The maximum A_{L1} that is ever allowable is obtained for maximum reflection
of L_{0}, i.e. when R_{L0} = 1. In this case, also the highest possible
effective reflection from L_{0} is obtained. Thus we can define a maximum
for the absorption in L_{1} that is still allowed as follows:
$${\mathrm{A}}_{\mathrm{L}\mathrm{1}\mathrm{max}}\mathrm{=}\mathrm{1}\mathrm{}{\mathrm{R}}_{\mathrm{min}}\mathrm{}\mathrm{\surd}\left({\mathrm{R}}_{\mathrm{min}}\right)$$
The choice R_{L0} = 1 implies that it is impossible to write data into
L_{0} since no absorption of optical radiation occurs. This extreme situation
would e.g. be applicable to a dualstack recordableROM disc or recordable L_{1},
ROM L_{0} disc.
In an embodiment A_{L1} ≤ A_{L0}.
In order to be able to record information via optical means in L_{0}, the
L_{0} stack should have a finite optical absorption at the wavelength of
the radiation beam, e.g. a laser. Since only part of the light of the recording
laser is transmitted through L_{1}, L_{0} should preferably be made
more sensitive, i.e. have a higher absorption than L_{1}, in order to keep
the required writepower within acceptable limits. For a recordable duallayer disc
it seems natural to impose the following two conditions: (i) same effective reflection
of both layers (same signal amplitudes which is preferred from drive pointofview)
and (ii) same effective absorption in both layers (same writepowers needed irrespective
of layer). These two boundary conditions give rise to a preferred absorption in
L_{1} that is given by:
$${\mathrm{A}}_{\mathrm{1}\mathrm{pref}}\mathrm{=}\mathrm{1}\mathrm{}3{\mathrm{R}}_{\mathrm{min}}\mathrm{/}41/41/4\cdot {\left[{\left(1,,{\mathrm{R}}_{\mathrm{min}}\right)}^{2},+,8,,{\mathrm{R}}_{\mathrm{min}}\right]}^{1/2}$$
Then, the preferred absorption in L_{0} (assuming T_{L0} = 0) is
given by
$${\mathrm{A}}_{\mathrm{0}\mathrm{pref}}\mathrm{=}\mathrm{1}\mathrm{}{\mathrm{R}}_{\mathrm{min}}\mathrm{/}{\left\{\mathrm{1},\mathrm{/},\mathrm{4},\mathrm{},{\mathrm{R}}_{\mathrm{min}},\mathrm{/},\mathrm{4},\mathrm{+},\mathrm{1},\mathrm{/},\mathrm{4},\mathrm{\cdot},{\left[{\left(\mathrm{1},\mathrm{},{\mathrm{R}}_{\mathrm{min}}\right)}^{\mathrm{2}},\mathrm{+},\mathrm{8},,{\mathrm{R}}_{\mathrm{min}}\right]}^{\mathrm{1}\mathrm{/}\mathrm{2}}\right\}}^{\mathrm{2}}$$
The next step is to recognize that the absorption in L_{0} and L_{1}
is mainly determined by the thickness of the recording layer d_{L} in L_{0}
and L_{1} respectively and the absorption coefficient k_{L&lgr;}
of the recording layer material in L_{0} and L_{1} respectively
(k_{L&lgr;} is the imaginary part of the complex refractive index n_{L&lgr;}).
To estimate the absorption within the recording stack the effect of a possible duallayer
stack design is omitted, which implies the following simplifications: (i) interference
effects within the recording layer are neglected, (ii) possible absorption in additional
layers that may be present is neglected, (iii) recording layer is embedded in between
two semiinfinite media having complex refractive index n0 and n2, see Fig.5. Typically
the upper surrounding medium will be transparent (substrate for L_{1} and
spacer for L_{0}) while the lower medium will be either transparent (spacer
for L_{1}) or highly reflecting (mirror for L_{0}). Then, the absorption
of optical power within this layer depends exponentially on both d_{L} and
k_{L} and is calculated to be:
$$\mathrm{A}\mathrm{=}\left[\mathrm{1},\mathrm{},\mathrm{exp},\left[\frac{\mathrm{}\mathrm{4}{\mathrm{\&pgr;d}}_{\mathrm{L}}{\mathrm{k}}_{\mathrm{L}}}{\mathrm{\&lgr;}},\mathrm{.},\left[\mathrm{1},\mathrm{+},\left(\left\frac{{\mathrm{n}}_{\mathrm{L}}\mathrm{}{\mathrm{n}}_{\mathrm{2}}}{{\mathrm{n}}_{\mathrm{L}}\mathrm{+}{\mathrm{n}}_{\mathrm{2}}}\right\right)\right]\right]\right]\mathrm{\cdot}\mathrm{[}\mathrm{1}\mathrm{}{\left(\left\frac{{\mathrm{n}}_{\mathrm{0}}\mathrm{}{\mathrm{n}}_{\mathrm{L}}}{{\mathrm{n}}_{\mathrm{0}}\mathrm{+}{\mathrm{n}}_{\mathrm{L}}}\right\right)}^{\mathrm{2}}]$$
&lgr; is the wavelength of the laser. The term (1+(n_{L}n_{2})/(n_{L}+n_{2}))
in the exponent is a measure for the effective thickness increase due to the portion
of light that is reflected back at the second interface of the recording layer,
see Fig. 5. The multiplicationfactor (1  (n_{L}n_{0})/(n_{L}+n_{0})^{2})
accounts for the light that is reflected at the first interface.
Typically, the L_{1} stack will be tuned for both
finite reflection and transmission. Then, the most dominant contribution to the
stack's absorption will be the absorption for a singlepass of light. The L_{0}
stack will be tuned for high reflection, and the stack's absorption will be close
that for a doublepass of light.
Preferably 1.5A_{L1}≤ A_{L0} ≤
2.5A_{L1}. From Fig.4 it can be seen that for equal writepower in L_{0}
and L_{1}, the absorption in L_{0} should typically be approximately
twice that of L_{1}. For the range of most interest, i.e. finite absorption
to achieve high T in L_{1} and high R in L_{0}, the double pass
will yield approximately twice as much absorption. Thus, in order to have the absorption
of both layers in the required range, the following is valid for both layers:
$$\mathrm{0.5}\mathrm{*}{\mathrm{A}}_{\mathrm{L}\mathrm{0}\mathrm{max}}\mathrm{\approx}\mathrm{Al}\mathrm{1}\mathrm{max}\mathrm{=}\mathrm{1}\mathrm{}{\mathrm{R}}_{\mathrm{min}}\mathrm{}\mathrm{\surd}\left({\mathrm{R}}_{\mathrm{min}}\right)\mathrm{\le}\mathrm{1}\mathrm{}\mathrm{exp}\left(\mathrm{},\mathrm{4},,{\mathrm{\&pgr;k}}_{\mathrm{L}},,{\mathrm{d}}_{\mathrm{L}},\mathrm{/},\mathrm{\&lgr;}\right)$$
From Fig.6 it can be seen that this approximation is best for the L_{1}
type stacks, where of course interference effects play a less important role.
One effect that is not taken into account in the above
calculations is the presence of the guide grooves in the medium, which are normally
present for tracking purposes in each recording stack adjacent the recording layer.
Due to these grooves, the radiation beam is diffracted and only a part (or none)
of the diffracted light is captured by the reflection/transmission measurement setup.
Thus the diffraction appears like a kind of absorption. The diffraction is used
to generate tracking signals like pushpull and trackcross and preferably these
signals are of equal magnitude on both stacks to minimize adjustments to the servosystems
when switching between the stacks. This in turn means that for both layers a similar
amount of light is lost in the reflection/transmission measurement. It means that
the indicated ranges of absorption and k/d range are really the upperlimit that
is allowed since the range is derived assuming no diffraction losses at all.
For the recordable type L_{1} recording layer having
a complex refractive index ñ_{L1&lgr;} = n_{L1&lgr;} 
i*k_{L1&lgr;} at the wavelength &lgr; and having a thickness d_{L1},
the following formula is fulfilled:
 k_{L1&lgr;} ≤ {&lgr;*ln[1/(R_{min} + √(R_{min}))]}/(4&pgr;*
d_{L1}) in which formula k_{L1&lgr;} is the absorption coefficient
of the L_{1} recording layer.
For the recordable type L_{0} recording layer having
a complex refractive index ñ_{L0&lgr;}= n_{L0&lgr;} i*k_{L0&lgr;}
at the wavelength &lgr; and having a thickness d_{L0}, the following formula
is fulfilled:
 k_{L0&lgr;}≤ {&lgr;*1n[1/(R_{min} + √(R_{min}))]}/(4&pgr;*
d_{L0}) in which formula K_{L0&lgr;} is the absorption coefficient
of the L_{0} recording layer.
It is noted that the above analysis is more accurate for
low kvalues (k<1); for k>1 the presented formula becomes inaccurate although
it still can serve as a rough estimate. Further it should be noted that the definition
of the thickness d_{L0} and d_{L1} of the recording layers requires
some further explanation. It may e.g. be that the recording layer thickness in a
guide groove is different from the thickness in between guide grooves due to leveling
effects during the application of the recording layer by e.g. spincoating. Hence
the thickness of the recording layer is defined as being the thickness where the
focused radiation beam spot is present during recording and read out.
To obtain a recordable dualstack optical data storage
medium that meets the specifications of the duallayer (stack) DVDROM disc, it
is required that &lgr; is about 655 nm, R_{min}= 0.18 and that K_{L0&lgr;}
and K_{L1&lgr;} fulfil the the requirements of equations Eq(9) and Eq(10).
It is a further object of the invention to provide a method
of manufacturing a dualstack optical data storage medium for recording using a
focused radiation beam having a wavelength &lgr; and entering through an entrance
face of the medium during recording, the stacks of the medium having an effective
optical reflection level of more than a specified value R_{min}, the medium
comprising
at least one substrate, the method comprising the steps of depositing on the at
least one substrate:
 a first recording stack L_{0}, comprising a recordable type L_{0}
recording layer, said first recording stack L_{0} having an optical reflection
value R_{L0} and an optical absorption value A_{L0} at the wavelength
&lgr;,
 a second recording stack named L_{1} comprising a recordable type L_{1}
recording layer, said second recording stack L_{1} having an optical reflection
value R_{L1} and an optical absorption value A_{L1} at the wavelength
&lgr;, said second recording stack being present closer to the entrance face than
the first recording stack,
 a transparent spacer layer sandwiched between the recording stacks, said transparent
spacer layer having a thickness substantially larger than the depth of focus of
the focused radiation beam, the which medium has an effective optical reflection
level of both the L_{0} stack and the L_{1} of more than a specified
value R_{min}.
This object has been achieved in accordance with the invention
by a method as described in the previous paragraph which is characterized in that
A_{L1} ≤1  R_{min}  √(R_{min}/R_{L0})
in which formula R_{min} is the minimum required effective optical reflection
value for each recording stack.
To obtain a recordable dualstack optical data storage
medium that meets the (expected) specifications of the dualstack Bluray Disc (BD),
it is required that &lgr; is about 405 nm, R_{min}= 0.04 and that k_{L0&lgr;}
and k_{L1&lgr;} fulfil the the requirements of equations Eq(9) and Eq(10).
For a dualstack Bluray Disc which is compatible with
the single layer reflection specification, it is required that &lgr; is about
405 nm, R_{min}= 0.12 and that k_{L0&lgr;} and k_{L1&lgr;}
fulfil the requirements of equations Eq(9) and Eq(10). Preferably 0.7*d_{L0}
< d_{L1} < 1.3*d_{L0} for the media described the last three
paragraphs.
It should be noted that the invention is not limited to
a single sided dual stack medium but that by varying substrate thicknesses e.g.
two single sided dual stack media according to the invention may be bonded together
forming a dual sided dual stack medium, which fulfils thickness requirements.
The invention will be elucidated in greater detail with
reference to the accompanying drawings, in which
Fig. 1 shows a schematic layout of a dualstack optical
data storage medium according to the invention. The effective reflection of both
stacks is indicated.
Fig. 2 shows the maximally allowable absorption in L_{1}
as a function of the imposed minimum effective reflectivity of both layers of the
dualstack disc.,
Fig. 3 shows the preferred absorption in L_{0}
and L_{1} compared to maximally allowable absorption in L_{1} as
a function of the effective reflectivity of L_{0} and L_{1}.
Fig. 4 shows the ratio between optical absorption in L_{0}
and L_{1} as a function of effective reflection.
Fig. 5 shows a schematic layout of the absorption of an
optical radiation beam by an absorbing recording layer, neglecting interference
effects within the recording layer.
Fig. 6 shows a comparison between calculated absorption
and approximation of Eq (9) for L_{1} type of stack (left) and L_{0}
type of stack (right). Solid line: exact calculation; dashed line: approximation.
Fig. 7 shows the maximum value of allowed k versus L_{1}recording
layer thickness for various values of effective reflection in case of a laser wavelength
within the DVD specification.
Fig. 8 shows the range of allowed kvalues as a function
of L_{1}recording layer thickness for a dualstack medium that meets the
DVD specifications (laser wavelength 655 nm, Rmin = 18 %).
Fig. 9 shows the maximally allowed kvalue in the case
of a DVDcompatible (for R=18%) and a BDcompatible (for R=4%) duallayer disc.
In Fig.1 a dualstack optical data storage medium 10 for
recording using a focused radiation beam, e.g. a laser beam 9, having a wavelength
&lgr;, is shown. The laser beam enters through an entrance face 8 of the medium
10 during recording. The medium 10 comprises substrates 1 and 7 with present on
a side thereof a first recording stack 2 named L_{0}, having an optical
reflection value R_{L0} and an optical absorption value A_{L0} at
the wavelength &lgr; and a second recording stack 5 named L_{1} having
an optical reflection value R_{L1} and an optical absorption value A_{L1}
at the wavelength &lgr;,
 a transparent spacer layer 4 is sandwiched between the recording stacks 2 and
5, said transparent spacer layer 4 having a thickness of 50 µm which is substantially
larger than the depth of focus of the focused laser beam 9. The absorption value
fulfils the following equation:
A_{L1} ≤ 1  R_{min}  √(R_{min}/R_{L0})
in which formula R_{min} is the minimum required effective optical reflection
value for each recording stack.
The first recording stack 2, comprises a recordable type
L_{0} recording layer 3, e.g. an azo dye or any other suitable dye. A guide
groove is present in the first substrate 1 or in the spacer layer 4, a first highly
reflective layer is present between the L_{0} recording layer 3 and the
substrate 1. A second substrate 7 is present with on a side thereof a second recording
stack 5 comprising a recordable type L_{1} recording layer 6, e.g. an azo
dye or any other suitable dye. The second L_{1} recording stack 5 is present
at a position closer to the entrance face 8 than the L_{0} recording stack
2. A second guide groove is present in the second substrate 7 or in the spacer layer
4. The first substrate 1 with L_{0} is attached to the substrate with L_{1}
by means of the transparent spacer layer 4, which may act as bonding layer. Specific
suitable L_{0}/L_{1} stacks designs are described below.
Embodiment 1 DVD recordable dual stack R
_{
min
} = 0.18, &lgr; = 655 nm, (layers in this order):
 Substrate 1 made of PC having a thickness of 0.60 mm
 Reflective layer of 100 nm Ag (n = 0.165.34i), Au, Cu or Al, or alloys thereof,
may be used as well,
 L_{0} recording layer 3 of an azo dye, with thickness of 80 nm, the
refractive index of the dye at a radiation beam wavelength of 655 nm is 2.24  0.02i.
 First semitransparent reflective layer made of Ag having a thickness of 10 nm,
Au, Cu or Al, or alloys may be used as well,
 Spacer layer 4 made of a transparent UV curable resin having having a thickness
of 50 µm,
 Second semitransparent reflective layer made of Ag having a thickness of 10
nm, Au, Cu or Al, or alloys may be used as well,
 L_{1} recording layer 6 of an azo dye, with thickness of 80 nm, the
refractive index of the dye at a radiation beam wavelength of 655 nm is 2.24  0.02i.
 Substrate 7 made of PC having a thickness of 0.58 mm
This stack design has the following reflection, absorption and transmission values:
 A_{L0} = 0.4
 A_{L1} = 0.2
 R_{L0} = 0.6
 R_{L1} = 0.2
 T_{L1} = 0.6
 T_{L0} = 0
The formula A_{L1} ≤ 1  R_{min}  √(R_{min}/RL_{0})
= 1 0.18  √(0.18/0.6) = 0.27 has been fulfilled. Furthermore K_{L0&lgr;}*
d_{L0} = 1.6 nm ≤ {&lgr;*1n[1/(R_{min} + √(R_{min}))]}/(4&pgr;)
= 26.4 nm and k_{L1&lgr;}* d_{L1}= 1.6 nm ≤ {&lgr;*ln[1/(R_{min}
+ √(R_{min}))]}/(4&pgr;) = 26.4 nm.
The first semitransparent reflective layer may also be
a SiO_{2} layer with a thickness of e.g. 20 nm; other dielectrics may be
used as well. In a different embodiment the first semitransparent reflective layer
may be absent. Furthermore, additional dielectric layers may be present between
the recording layer and the reflective and/or semitransparent reflective layers.
The second semitransparent may also be a dielectric (e.g. Si02) or semiconducting
(e.g. Si) layer. Furthermore, additional dielectric layers may be present between
the recording layer and the second semitransparent reflective layer and/or between
second semitransparent reflective layer and the spacer layer and/or between the
recording layer and the substrate 7.
The following example does not form part of the claimed
invention but represents background art that is useful for understanding the invention.
Example: BD recordable dual stack R
_{
min
} = 0.12, &lgr; = 405 nm (layers in this order):
 Substrate 1 made of PC having a thickness of 1.1 mm
 Reflective layer of 100 nm Ag (n = 0.172i), Au, Cu or Al, may be used as well,
 L_{0} recording layer 3 of an organic dye, with thickness of 50 nm,
the refractive index of the dye at a radiation beam wavelength of 405 nm is 2.4
 0.04i.
 First transparent dielectric layer made of Si02 having a thickness of 20 nm,
other dielectrics (Si3N4, ZnSSiO2, Al2O3, A1N) may be used as well,
 Spacer layer 4 made of a transparent UV curable resin having a thickness of
25 µm,
 L_{1} recording layer 6 of an organic dye, with thickness of 50 nm,
the refractive index of the dye at a radiation beam wavelength of 405 nm is 2.4
 0.04i.
 Second transparent dielectric layer made of SiO_{2} having a thickness
of 20 nm, other dielectrics (Si3N4, ZnSSi02, Al2O3, AlN) may be used as well.
 Substrate 7, in this embodiment also called cover layer, made of a transparent
UV curable resin, having a thickness of 0.075 mm.
This stack design has the following reflection, absorption and transmission values:
 A_{L0} = 0.6
 A_{L1} =0.2
 R_{L0} = 0.4
 R_{L1} = 0.2
 T_{L1} = 0.6
 T_{L0} = 0
The formula A_{L1} ≤ 1  R_{min}  √(R_{min}/R_{L0})
= 1 0.12  √(0.12/0.4) = 0.33 has been fulfilled. Furthermore K_{L0&lgr;}*
d_{L0} = 2 nm ≤ {&lgr;*1n[1/(R_{min} + √(R_{min}))]}/(4&pgr;)
= 24 nm and k_{L1&lgr;}* d_{L1} = 2 nm ≤ {&lgr;*ln[1/(R_{min}
+ √(R_{min}))]}/(4&pgr;) = 24 nm
In Fig.2 a graph is drawn representing the maximum allowable
absorption in L_{1} A_{L1max} as a function of a minimum effective
reflection R_{min} of both recording stacks L_{0} and L_{1}.
Note that the maximum achievable value of R_{min} is about 0.38. This value
represents the case in which the L_{1} stack does not have absorbance anymore
and hence recording is not possible, while also the L_{0} stack has no absorption
and maximum reflection (R_{L0} =1).
In Fig. 3 the preferred absorption in L_{0} and
L_{1} are compared to the maximally allowable absorption in L_{1}
as a function of the effective reflectivity of L_{0} and L_{1}.
This preferred absorption graphs are representations of equations (6) and (7).
In Fig. 4 the ratio between A_{L0} and A
_{L1} is shown as a function of the effective reflectivity of L_{0}
and L_{1}. It can be seen that preferably this ratio is in the range 1.5
 2.5 more preferably in the range 1.5  2.0.
In Fig. 5 a schematic layout of a recording layer 3, 6
in a dual stack optical data storage medium 10 is shown (see Fig. 1). The path of
an optical radiation beam is show. The absorption in L_{0} and L_{1}
is mainly determined by the thickness of the recording layer d_{L} and the
absorption coefficient k_{L&lgr;} of the recording layer material (k_{L&lgr;}
is the imaginary part of the complex refractive index n_{L&lgr;}). To
estimate the absorption within the recording stack the detailed effect of a possible
duallayer stack design is omitted, which implies the following simplifications:
(i) interference effects within the recording layer are neglected, (ii) possible
absorption in additional layers that may be present is neglected, (iii) recording
layer is embedded in between two semiinfinite media having complex refractive index
n0 and n2. Typically, the upper surrounding medium will be transparent (substrate
for L_{1} and spacer for L_{0}) while the lower medium will be either
transparent (spacer for L_{1}) or highly reflecting (mirror for L_{0}).
Then, the absorption of the optical radiation beam within this layer depends exponentially
on both d_{L} and k_{L}, represented by equation (8).
In Fig.6 modeling results are presented of the absorption
as a function of the recording layer thickness. The solid line indicates the exact
calculation while the dashed line is the approximation of equation (9). Notice that
the approximation is best for the L_{1} stack and reasonable for the L_{0}
stack.
In Fig. 7 the maximum allowed k value for the recording
layer of L_{1} is shown as a function of the recording layer thickness d_{L1}
for various values of the R_{min}.
In Fig. 8 the special case where R_{min} =0.18
is drawn separately where it the area with allowed kvalues has been hatched.
In Fig. 9 the same is done and the graph for BD has been
added as a comparison.
It should be noted that the abovementioned embodiments
illustrate rather than limit the invention, and that those skilled in the art will
be able to design many alternative embodiments without departing from the scope
of the appended claims. In the claims, any reference signs placed between parentheses
shall not be construed as limiting the claim. The word "comprising" does not exclude
the presence of elements or steps other than those listed in a claim. The word "a"
or "an" preceding an element does not exclude the presence of a plurality of such
elements. The mere fact that certain measures are recited in mutually different
dependent claims does not indicate that a combination of these measures cannot be
used to advantage.
According to the invention a dualstack optical data storage
medium for recording using a focused radiation beam having a wavelength &lgr;
is described. The beam enters through an entrance face of the medium during recording.
The medium comprises at least one substrate with present on a side thereof a first
recording stack named L_{0}, comprising a recordable type L_{0}
recording layer, said first recording stack L_{0} having an optical reflection
value R_{L0} and an optical absorption value A_{L0} at the wavelength
&lgr;, and a second recording stack named L_{1} comprising a recordable
type L_{1} recording layer, said second recording stack L_{1} having
an optical reflection value R_{L1} and an optical absorption value A_{L1}
at the wavelength &lgr;, and a transparent spacer layer sandwiched between the
recording stacks. By fulfilling the formula A_{L1} ≤ 1  R_{min}
 √(R_{min}/R_{L0}) in which formula R_{min} is the
minimum required effective optical reflection value for each recording stack full
compatibility is achieved with a read only (ROM) version of the medium.

Anspruch[de] 
Optisches Datenspeichermedium (10) mit zwei Schichtenfolgen (DualStackDatenspeichermedium)
zum Aufzeichnen unter Verwendung eines fokussierten Strahlenbündels (9), das
eine Wellenlänge &lgr; von etwa 655 nm hat und während des Aufzeichnens
durch eine Eintrittsfläche (8) des Mediums (10) eintritt, umfassend:
 mindestens ein Substrat (1, 7), wobei auf einer Seite hiervon Folgendes
vorhanden ist:
 eine erste Aufzeichnungsschichtenfolge (2) L_{0}, die eine
Aufzeichnungsschicht (3) des beschreibbaren Typs umfasst, wobei die erste Aufzeichnungsschichtenfolge
L_{0} bei der Wellenlänge &lgr; einen Wert R_{L0} der optischen
Reflexion und einen Wert A_{L0} der optischen Absorption hat und die Aufzeichnungsschicht
(3) des beschreibbaren Typs bei der Wellenlänge &lgr; einen komplexen Brechungsindex
ñ_{L0&lgr;} = n_{L0&lgr;}  1*k_{L0&lgr;} und
eine Dicke d_{L0} hat,
 eine zweite Aufzeichnungsschichtenfolge (5) L_{1}, die eine
Aufzeichnungsschicht (6) des beschreibbaren Typs umfasst, wobei die zweite Aufzeichnungsschichtenfolge
L_{1} bei der Wellenlänge &lgr; einen Wert R_{L1} der optischen
Reflexion und einen Wert A_{L1} der optischen Absorption hat und die Aufzeichnungsschicht
(6) des beschreibbaren Typs bei der Wellenlänge &lgr; einen komplexen Brechungsindex
ñ_{L1&lgr;} = n_{L1&lgr;}  i*k_{L1&lgr;} und
eine Dicke d_{L1} hat, wobei die zweite Aufzeichnungsschichtenfolge näher
an der Eintrittsfläche liegt als die erste Aufzeichnungsschichtenfolge,
 eine transparente Trennschicht (4), die sandwichartig zwischen den
Aufzeichnungsschichtenfolgen (2, 5) angeordnet ist, wobei die transparente Trennschicht
(4) eine Dicke hat, die wesentlich größer ist als die Brennpunkttiefe
des fokussierten Strahlenbündels (9),
dadurch gekennzeichnet, dass
${\mathrm{k}}_{\mathrm{L}\mathrm{1}}\mathrm{\le}\mathrm{1}{\mathrm{R}}_{\mathrm{min}}\sqrt{\left({\mathrm{R}}_{\mathrm{min}},/,{\mathrm{R}}_{\mathrm{L}\mathrm{0}}\right)}$
ist, wobei in der Formel R_{min} = 0,18 ist, und dass die folgenden Formeln
erfüllt sind:
$${\mathrm{k}}_{\mathrm{L}\mathrm{0}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},\mathrm{+},\sqrt{\left({\mathrm{R}}_{\mathrm{min}}\right)}\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{0}}\right)$$
und
$${\mathrm{k}}_{\mathrm{L}\mathrm{1}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},\mathrm{+},\sqrt{\left({\mathrm{R}}_{\mathrm{min}}\right)}\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{1}}\right)\mathrm{.}$$
Optisches DualStackDatenspeichermedium (10) nach Anspruch 1, wobei
A_{L1} ≤ A_{L0} ist.
Optisches DualStackDatenspeichermedium (10) nach Anspruch 2, wobei
1,5A_{L1} ≤ A_{L0} ≤ 2,5A_{L1} ist.

Anspruch[en] 
A dualstack optical data storage medium (10) for recording using a
focused radiation beam (9) having a wavelength &lgr;, of about 655 nm and entering
through an entrance face (8) of the medium (10) during recording, comprising:
 at least one substrate (1, 7) with present on a side thereof:
 a first recording stack (2) L_{0}, comprising a recordable
type recording layer (3), said first recording stack L_{0} having an optical
reflection value R_{L0} and an optical absorption value A_{L0} at
the wavelength &lgr;, and the recordable type recording layer (3) has a complex
refractive index ñ_{L0&lgr;} = n_{L0&lgr;}  i*k_{L0&lgr;}
at the wavelength &lgr; and has a thickness d_{L0},
 a second recording stack (5) L_{1} comprising a recordable
type recording layer (6), said second recording stack L_{1} having an optical
reflection value R_{L1} and an optical absorption value A_{L1} at
the wavelength &lgr;, and the recordable type recording layer (6) has a complex
refractive index ñ_{L1&lgr;} = n_{L1&lgr;} i*k_{L1&lgr;}
at the wavelength &lgr; and has a thickness d_{L1}, said second recording
stack being present closer to the entrance face than the first recording stack,
 a transparent spacer layer (4) sandwiched between the recording stacks
(2, 5), said transparent spacer layer (4) having a thickness substantially larger
than the depth of focus of the focused radiation beam (9),
characterized in that A_{L1} ≤ 1  R_{min}  √(R_{min}/R_{L0})
in which formula R_{min} = 0.18 and in that the following formulas
are fulfilled:
$${\mathrm{k}}_{\mathrm{L}\mathrm{0}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},\mathrm{+},\mathrm{\surd},\left({\mathrm{R}}_{\mathrm{min}}\right)\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{0}}\right)$$
and
$${\mathrm{k}}_{\mathrm{L}\mathrm{1}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},+,\surd ,\mathrm{\left(},{\mathrm{R}}_{\mathrm{min}},\mathrm{\right)}\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{1}}\right)\mathrm{.}$$
A dualstack optical data storage medium (10) according to claim 1,
wherein A_{L1} ≤ A_{L0}.
A dualstack optical data storage medium (10) according to claim 2,
wherein 1.5A_{L1}≤ A_{L0}≤2.5A_{L1}.

Anspruch[fr] 
Support de stockage optique de données à double empilement
(10) pour enregistrement en utilisant un faisceau de rayonnement focalisé (9)
qui a une longueur d'onde &lgr; d'environ 655 nm et pénètre par une
face d'entrée (8) du support (10) pendant l'enregistrement, comprenant :
 au moins un substrat (1, 7) sur un côté duquel sont présents
:
 un premier empilement d'enregistrement (2) L_{0}, qui comprend
une couche d'enregistrement de type enregistrable (3), ledit premier empilement
d'enregistrement L_{0} ayant une valeur de réflexion optique R_{L0}
et une valeur d'absorption optique A_{L0} à la longueur d'onde &lgr;,
et la couche d'enregistrement de type enregistrable (3) a un indice de réfraction
complexe ñ_{L0&lgr;} = n_{L0&lgr;}  i*k_{L0&lgr;}
à la longueur d'onde &lgr; et a une épaisseur d_{L0};
 un deuxième empilement d'enregistrement (5) L_{1}, qui
comprend une couche d'enregistrement L_{1} de type enregistrable (6), ledit
deuxième empilement d'enregistrement L_{1} ayant une valeur de réflexion
optique R_{L1} et une valeur d'absorption optique A_{L1} à
la longueur d'onde &lgr;, et la couche d'enregistrement de type enregistrable
(6) a un indice de réfraction complexe ñ_{L1&lgr;} = n_{L1&lgr;}
 1*k_{L1&lgr;} à la longueur d'onde &lgr; et a une épaisseur
d_{L1}, ledit deuxième empilement d'enregistrement étant présent
plus près de la face d'entrée que le premier empilement d'enregistrement;
 une couche d'espacement transparente (4) prise en sandwich entre les
empilements d'enregistrement (2, 5), ladite couche d'espacement transparente (4)
ayant une épaisseur essentiellement plus grande que la profondeur de foyer
du faisceau de rayonnement focalisé (9),
caractérisé en ce que A_{L1} ≤ 1  R_{min}
 √(R_{min} / R_{L0}), formule dans laquelle R_{min}
= 0,18, et en ce que les formules suivantes sont satisfaites :
$${\mathrm{k}}_{\mathrm{L}\mathrm{0}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},\mathrm{+},\mathrm{\surd},\left({\mathrm{R}}_{\mathrm{min}}\right)\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{0}}\right)$$
et
$${\mathrm{k}}_{\mathrm{L}\mathrm{1}\mathrm{\&lgr;}}\mathrm{\le}\left\{\mathrm{\&lgr;},\mathrm{*},\mathrm{ln},,\left[\mathrm{1},\mathrm{/},\left({\mathrm{R}}_{\mathrm{min}},\mathrm{+},\mathrm{\surd},\left({\mathrm{R}}_{\mathrm{min}}\right)\right)\right]\right\}\mathrm{/}\left(\mathrm{4},,\mathrm{\&pgr;},\mathrm{*},{\mathrm{d}}_{\mathrm{L}\mathrm{1}}\right)$$
Support d'enregistrement optique de données à double empilement
(10) selon la revendication 1, dans lequel A_{L1} ≤ A_{L0}.
Support d'enregistrement optique de données à double empilement
(10) selon la revendication 2, dans lequel 1,5 A_{L1} ≤ A_{L0}
≤ 2,5 A_{L1}.


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