System and method for simultaneously verifying optical data writing and optical data writing medium for use therein

The optical disk writing system disclosed is one in which data written on an optical data writing medium is verified essentially simultaneously with writing on the medium. The written data is verified by detecting a timing of the light irradiated on the medium and by measuring return light doses at rise and fall of the light irradiated on the medium and comparing the doses thereof with predetermined reference values. The medium uses a phase transition reversible between a crystal state and an amorphous state for writing/reading/erasing of data on a data writing film, and includes a first protective film, a phase transition type data writing film, a second protective film, and a reflection film, formed in this order, on a transparent substrate. The thickness of the first protective film is set such that, of three reflectivities of the data writing film respectively in crystal, amorphous and melted states, the reflectivity in the crystal state and the reflectivity in the melted state are made different from each other, and the reflectivity in the amorphous state and the reflectivity in the melted state are made different from each other. A high speed writing operation can be carried out and an overwrite operation can be completed in one disk rotation time.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to an optical disk writing system and method 
for writing, with which writing and reading of data is made by utilizing a 
phase transition optical disk. More particularly, the invention relates to 
a system and method for simultaneously verifying data written on an 
optical data writing medium, with which the data in high density is 
written, read and erased by using light. The invention also relates to an 
optical data writing medium in which there are changes in structures and 
optical characteristics between a crystal state and an amorphous state 
depending on differences in temperature rise due to light irradiation and 
thermal hysteresis due to temperature fall. Thus, the invention relates to 
a phase transition optical disk and simultaneous writing verifying method. 
(2) Description of the Related Art 
In one prior art optical disk writing system and writing method, data is 
written on an optical disk by irradiating the disk with a laser beam. When 
writing data into the optical disk, it is necessary to verify whether the 
data being written has correctly been written on the optical disk. 
Generally, this verification is made by reading the written area after 
completion of the writing. 
In the above prior art, a plurality of beam spots are formed. For the 
formation of the beam spots, diffraction grating is used as disclosed in 
Japanese Patent Application Kokai Publication No. Hei 2-9024 or Japanese 
Patent Application Kokai Publication No. Hei 3-41632, or a plurality of 
light sources are used. The verification is made using one or two of these 
beam spots. 
With respect to a magneto-optical disk, there is a proposal disclosed in 
Japanese Patent Application Kokai Publication No. Hei 5-144112 in which a 
single beam verification using a single beam spot is realized by utilizing 
characteristics inherent to a magneto-optical disk. 
However, in the prior art verification methods applied to the optical disk 
systems described above, there have been problems in that, when 
verification is made after the writing operation, the time required for 
the writing is long and, when a plurality of beam spots are used, there 
have been problems in that cost is high and adjustment of the optical 
system is difficult. 
An optical disk writing system using laser light makes it possible to 
perform large volume data writing and non-contact high speed access, and 
thus practical use thereof as large volume memory is increasing. Optical 
disks are grouped into dedicated reading type such as compact and laser 
disks, write once read many type in which a user can carry out writing, 
and rewritable type in which a user can write and erase repeatedly. The 
write once read many type disk and the rewritable type disk are used as 
external memories of computers or document/image files. 
The rewritable type disk includes the phase transition type optical disk 
which utilizes a phase transition of a writing film and the 
magneto-optical disk which utilizes a magnetization direction transition 
of a vertical magnetic film. Of these, the phase transition type optical 
disk does not require an external magnetic field and, moreover, since 
overwriting can be made easily, the optical disk of this type is expected 
to become a leading type of rewritable type of optical disk. 
A conventional rewritable type optical disk, i.e., a phase transition type 
optical disk utilizing a writing film in which the transition is caused to 
occur between a crystal state and an amorphous state by the laser light 
irradiation exists. In the phase transition type optical disk, writing is 
accomplished by irradiating on the writing film on a high power laser spot 
corresponding to the data to be written and locally raising writing film 
temperature thereby causing crystal/amorphous transition to occur, and 
reading is accomplished, using low power laser light, by reading changes 
in the resulting optical constant as an intensity difference of the 
reflected light or phase transition. For example, in the phase transition 
optical disk utilizing a writing film in which the crystallization time is 
comparatively slow, writing is accomplished by rotating the disk, 
irradiating the laser light on the writing film formed on the disk, 
raising the temperature of the writing film to above the melting point 
and, after the laser light has passed, rapidly cooling the writing film so 
as to change it into an amorphous state. During erasing, writing film 
temperature is held for a time sufficient to allow crystallization under a 
crystallization permitting temperature range which is above the 
crystallization temperature and below the melting point, whereby the 
writing film is crystallized. The known method for this is a method in 
which the light irradiated is in an oblong shape extending along the laser 
advancing direction. When pseudo-over-write is carried out using two beams 
for writing new data while data already recorded is being erased, an 
oblong laser for erasing is positioned for irradiation ahead of a circular 
laser for writing. 
On the other hand, in the disk utilizing the data writing film in which 
quick crystallization is possible, one laser beam focused into a circular 
shape is used. In the known method, change to a crystal state or an 
amorphous state is effected by changes between two levels of laser light 
power. That is, by irradiating on the writing film laser light having a 
power allowing writing film temperature to rise above the melting point, 
most portions thereof are changed to an amorphous state when cooled and, 
on the other hand, the portions which are irradiated by the laser light 
having a power for writing film temperature to rise above the 
crystallization temperature or to reach a temperature below the melting 
point are changed to a crystal state. For writing films of the phase 
transition type optical disks, the materials used include GeSbTe system, 
InSbTe system, InSe system, InTe system, AsTeGe system, TeOx-GeSn system, 
TeSeSn system, SbSeBi system, and BiSeGe system which are chalcogenide 
type materials. Film formation methods for all these materials include 
resistive heating vacuum vapor deposition, electron beam vacuum vapor 
deposition, and sputtering methods. The state of the writing film 
immediately after film formation is a kind of amorphous state and, for 
forming an amorphous state portion on the film upon writing, a formatting 
process is carried out for the entire writing film so as to be preset to a 
crystal state. The writing is achieved by forming an amorphous portion in 
the film of the crystal state. 
Conventionally, in order to carry out a verifying operation to confirm 
whether the erasing operation and writing operation have been correctly 
effected and also whether the recorded state is correct, time 
corresponding to three disk rotations is required. Also, in the phase 
transition optical disk which permits overwriting, two disk rotations, one 
for an overwrite writing operation and the other for a verifying operation 
are required. 
As above, when the data is rewritten, there is required a waiting time 
corresponding to at least two disk rotations and, for this reason, disk 
transfer speed is quite slow. 
As a method for compensating for lowering of transfer speed caused by disk 
rotation waiting time during writing, verification carried out during 
writing, that is, simultaneous writing verifying operation is effective. 
For example, in the write once read many type optical disk disclosed in 
Japanese Patent Application Kokai Publication No. Sho 55-89919, Japanese 
Patent Application Kokai Publication No. Sho 57-92438 and Japanese Patent 
Application Kokai Publication No. Sho 60-145537, a change in the amount of 
reflected light from the medium during writing is monitored so as to 
detect the written state of the data from reproduced wave shapes. 
Detection can be made because, by the writing, there occurs a shape change 
such as by formation of a hole in the write once and read many time 
medium, and the amount of the reflected light changes accordingly. 
Also, in the magneto optical disk, it has been proposed, as disclosed in 
the Japanese Patent Application Kokai Publication No. Hei 3-207040, that 
the amount of the reflected light be monitored and, from change in the 
Kerr rotation angle of the optical beam caused by writing, the verifying 
signal is detected and verifying is effected at the same time as writing. 
However, in the phase transition disk with which overwriting can easily be 
made, it is currently the case that no attempt has been made to effect the 
verifying operation during writing and no knowledge has been gained as to 
what structure of the disk is suited to verification during writing. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical disk writing 
system and method for writing, which utilizes an optical data writing 
medium of a type in which there is a transition in reflectivity or optical 
phase in a writing film when light is irradiated thereon, and which 
enables writing operation and verification to be effected concurrently by 
a single beam spot. 
Another object of the present invention is to provide a medium which is 
suited to simultaneous verifying operation during writing on the phase 
transition type optical disk. Also, the present invention aims to provide 
a writing/reading method which excels in simultaneous verification 
operation during writing. A further object of the invention is to provide 
a medium in which reflectivities in the melt state, crystal state and 
amorphous state are distinctly different, and a method which is suited to 
simultaneous verification during writing. 
According to a first aspect of the invention, there is provided an optical 
disk writing system in which predetermined data is written on an optical 
data writing medium and the written data is verified, the system 
comprising: 
a light spot irradiation means for forming a single light spot on the 
optical data writing medium; 
a return light dose measurement means for measuring a dose of return light 
of the light spot from the optical data wiring medium; and 
an irradiation light timing detection means for detecting rise and fall 
timing of the light irradiated on the optical data writing medium, 
the written data being verified by measuring the return light dose at the 
rise and fall timings of the light irradiated on the optical data writing 
medium. 
A method for writing data using the above optical disk includes the step of 
verifying the written data essentially simultaneously with writing of the 
data on the optical data writing medium, the verifying step including the 
step of detecting timing of the light irradiated on the optical data 
writing medium, the verifying being effected by measuring return light 
doses at rise and fall of the light irradiated on the optical data writing 
medium and comparing the doses thereof with predetermined reference 
values. 
According to a second aspect of the invention, there is provided an optical 
data writing medium which uses a phase transition reversible between a 
crystal state and an amorphous state for writing/reading/erasing of data 
on a data writing film, and which comprises: 
a transparent substrate; 
a first protective film formed on the transparent substrate; 
a phase transition type data writing film formed on the first protective 
film; 
a second protective film formed on the phase transition type data writing 
film; and 
a reflection film formed on the second protective film, 
the first protective film having a thickness being set such that, of three 
reflectivities of the data writing film respectively in a crystal state, 
in an amorphous state, and in a melted state, the reflectivity in the 
crystal state and the reflectivity in the melted state are rendered 
different from each other, and the reflectivity in the amorphous state and 
the reflectivity in the melted state are rendered different from each 
other. 
A method for verifying an optical data writing medium simultaneously with 
writing as explained above includes the step of detecting reflected light 
from the optical data writing medium when the data writing film formed on 
the optical data writing medium is changed to the melted state during the 
data writing by light irradiation.

PREFERRED EMBODIMENTS OF THE INVENTION 
Now, an optical disk writing system and method for writing, as an 
embodiment of the first aspect of the invention, are explained with 
reference to the drawings. FIG. 1 is referred to for describing a system 
and method embodied in the optical disk writing system and the method for 
writing according to this aspect of the invention. FIG. 1 diagrammatically 
shows the optical disk writing system. 
The optical disk system shown in FIG. 1 is constituted by laser light 
source 1, lenses 2 and 4, beam splitter 3, phase transition optical disk 
5, laser driver 6, differentiation circuit 8, delay circuit 9, comparators 
10, 11, 15 and 16, AND circuits 17 and 18, and pulse count circuit 21. 
The laser light source 1 comprises a light source system which emits a 
laser beam so as to produce a beam spot for writing/reading data on the 
phase transition optical disk 5. The laser light source 1 is driven by the 
laser driver 6. 
Lenses 2 and 4 constitute a collector which focuses the laser light emitted 
from the laser light source 1 as a laser beam spot on the phase transition 
optical disk 5. The beam splitter 3 comprises a laser beam divider, which 
is disposed in the optical axis of the incident light and divides and 
extracts the return light from the optical disk. In this embodiment of the 
invention, the light emitted from the laser light source 1 is allowed to 
advance linearly, and the reflected light from the phase transition 
optical disk 5 is divided and reflected after being refracted 90.degree.. 
The phase transition optical disk 5 is an optical disk for writing data by 
irradiation of the laser light. The writing of data is carried out by 
causing the states of the writing film of the optical disk to undergo 
transition in response to a local temperature rise due to light absorption 
of the laser light. The laser driver 6 is a circuit portion for driving 
the laser light source 1 and causing the laser light to be generated. 
The differentiation circuit 8 is a circuit for extracting a O/1 state 
transition point of an output signal from the laser driver 6 that drives 
the laser light source 1. The delay circuit 9 is a signal delay circuit 
portion for timings between the driving signal and the reflection signal. 
The comparators 10, 11, 15 and 16 comprise a comparison device for 
comparing the signal to be compared and the predetermined value. In this 
embodiment, by using these comparators, the transition states of the 
driving signal and the reflection signal are represented by pulse signals. 
The AND circuits 17 and 18 constitute a logical product circuitry, which 
finds transition states of the driving signal of the laser driver 6, the 
laser light emitted from the laser light source 1, and the reflection 
light from the phase transition optical disk 5. The transition states make 
it possible to confirm, that is to verify, emission of laser light and 
execution of writing. The pulse count circuit 21 is a circuit portion for 
confirming whether the write confirmation signal and the read confirmation 
signal are correctly present. By this confirmation, reliability of the 
verification can be enhanced. 
FIG. 2 shows, in timing charts, timings of verifying operations. The charts 
show, respectively, timings of the signals indicated in FIG. 1. 
Waveform 51 shows a laser light source driving signal; 
Waveform 52 shows a gate signal corresponding to a rise portion of the 
write signal; 
Waveform 53 shows a gate signal corresponding to a fall portion of the 
write signal; 
Waveform 54 shows an optical signal of the light returning from the phase 
transition optical disk during writing; 
Waveform 55 shows a comparator signal in which the return light dose 
monitor signal IL outputted when the writing signal has risen is compared 
with the reference voltage CLV; and 
Waveform 56 shows a comparator signal in which the return light dose 
monitor signal IT outputted when the writing signal has fallen is compared 
with the reference voltage. 
Further, by using the above mentioned signals, the output signal 57 of the 
AND circuit 17 is a logical product of the gate signal 52 with the 
waveform shown in FIG. 2 and the comparator signal 55 with the waveform 
shown in FIG. 2, and is represented by a symbol VL in the explanation 
given below. By this signal, it is possible to confirm that desired laser 
power has been irradiated on the writing film of the phase transition 
optical disk 5. 
Also, the output signal 58 of the AND circuit 18 is a logical product of 
the gate signal 53 with the waveform shown in FIG. 2 and the comparator 
signal 56 with the waveform shown in FIG. 2, and is represented by a 
symbol VT in the explanation given below. By this signal, it is possible 
to confirm that the writing film of the phase transition optical disk 5 
has melted and writing has correctly been accomplished. 
Furthermore, where the writing signal 61 delayed by the delay circuit 9 and 
the logical product signals VL and VT take the logical product again, 
verification precision can be enhanced. In this embodiment, the delayed 
writing signal 61 is made a gate signal, and the number of the pulses of 
the output signals 57 of the AND circuit 17 and the output signals 58 of 
the AND circuit 18, which are generated in the gate, are counted. In this 
counting, when both of the two pulses, namely, one pulse of the signal VL 
and one pulse of the signal VT, are counted, it confirms that writing has 
been correctly been accomplished. Here, the writing signal is delayed 
because the time for the laser light rising and the time necessary for the 
writing film to melt are taken into account. 
EXAMPLE 1 
The phase transition optical disk 5 of this example is structured as shown 
in FIG. 3. The structure was obtained by depositing ZnS--SiO.sub.2 102 to 
a thickness of 150 nm, Ge.sub.2 Sb.sub.2 Te.sub.5 103 to a thickness of 20 
nm, ZnS--SiO.sub.2 104 to 20 nm, and Al 105 to a thickness of 60 nm in 
this sequence, by sputtering, on a polycarbonate substrate 101. On the 
optical disk, (2-7)-modulated random data was written by rotating this 
phase transition optical disk 5 at a linear speed of 7.5 m/s and setting 
the writing pulse width to 50 ns. The writing power Pw was set to between 
14 and 16 mW, the erasing power Pe was set to 7 mW, and the reading power 
Pr was set to 1 mW. 
The reflectivities of the writing film were 25% when the film was in the 
crystal state, and 8% when in the amorphous state. The return light dose 
Ic from the phase transition optical disk 5, in a crystal state of the 
writing film, during reading was 1.5 V, and the return light dose IA from 
the phase transition optical disk 5 in an amorphous state was 0.48 V. The 
writing error signal generating times during writing, when CLV was set to 
(0.9.times.Ic.times.Pw)/Pr, while CTV was set to 
0.5.times.(Ic+IA).times.(PE)/Pr, are shown in FIG. 4. Shown in FIG. 4 are 
results of checking of writing errors by reading the written area after 
the writing as in the prior art. As is noted from FIG. 4, the detected 
number of writing error generation times in this example is in accord with 
that in the prior art example. 
In the same way as above, verifying was carried out by varying the 
reference voltage CLV and the reference voltage CTV, and checking was made 
as to optimum values of reference voltages CLV and CTV. As a result, it 
was found that, with respect to reference voltage CLV, writing error 
generating times detected were in accord with those in the prior art 
example when the condition (0.7.times.Ic.times.Pw)/Pr&lt;CLV(Ic.times.Pw)/Pr 
was satisfied. Also, with respect to reference voltage CTV, writing error 
generating times detected were in accord with those in the prior art 
example when the condition (IA.times.PE)/Pr&lt;CTV&lt;(Ic.times.PE)/Pr was 
satisfied. 
EXAMPLE 2 
The phase transition optical disk 5 of this example employs a structure 
which is obtained by depositing ZnS--SiO.sub.2 102 to a thickness of 100 
nm, Ge.sub.1 Sb.sub.2 Te.sub.4 103 to a thickness of 25 nm, ZnS--SiO.sub.2 
104 to a thickness of 20 nm, and Al 105 to a thickness of 60 nm, in this 
sequence, by sputtering on a polycarbonate substrate 101. On this optical 
disk, (2-7)-modulated random data was written by rotating the disk at a 
linear speed of 7.5 m/s and setting the writing pulse width to 50 ns. 
Writing power Pw was set to between 16 and 18 mW, erasing power Pe was set 
to 8 mW, and reading power Pr was set to 1 mW. 
The reflectivities of the writing film were 20% when the film was in the 
crystal state, and 6% when in the amorphous state. The return light dose 
Ic from the phase transition optical disk 5, in a crystal state of the 
writing film, during reading was 1.2 V, and the return light dose IA from 
the phase transition optical disk 5 in an amorphous state was 0.36 V. 
Writing error signal generating times with respect to writing power during 
writing, when the reference voltage CLV was set to 
(0.8.times.Ic.times.Pw)/Pr, while the reference voltage CTV was set to 
0.4.times.(Ic+IA).times.(Pe)/Pr are shown in FIG. 5. Shown in FIG. 5 are 
results of checking of writing errors by reading the written area after 
writing as in the prior art. As is noted from FIG. 5, the detected number 
of writing error generation times in this example is in accord with that 
in the prior art example. 
In the same way as above, verifying was carried out by varying the 
reference voltage CLV and the reference voltage CTV, and checking was made 
as to optimum values of the reference voltages CLV and CTV. As a result, 
it was found that, with respect to the reference voltage CLV, writing 
error generating times detected were in accord with those in the prior art 
example when the condition (0.7.times.Ic.times.Pw)/Pr&lt;CLV&lt;(Ic.times.Pw)/Pr 
was satisfied. Also, with respect to the reference voltage CTV, writing 
error generating times detected were in accord with those in the prior art 
example when the condition (IA.times.PE)/Pr&lt;CTV&lt;(Ic.times.PE)/Pr was 
satisfied. 
EXAMPLE 3 
The phase transition optical disk 5 of this example is structured as shown 
in FIG. 6. The structure was obtained by depositing Au 112 to a thickness 
of 10 nm, ZnS--SiO.sub.2 113 to a thickness of 150 nm, Ge.sub.2 Sb.sub.2 
Te.sub.5 114 to a thickness of 20 nm, ZnS--Si0.sub.2 115 to a thickness of 
45 nm, and Al 116 to a thickness of 100 nm, in this sequence, by 
sputtering on a polycarbonate substrate 111. On this optical disk 5, 
(2-7)-modulated random data was written by rotating the disk at a linear 
speed of 7.5 m/s and setting the writing pulse width to 50 ns. Writing 
power Pw was set to between 18 and 20 mW, erasing power Pe was set to 9 
mW, and reading power Pr was set to 1 mW. 
In the medium of this configuration, reflectivity in the amorphous state is 
higher, and reflectivity when the writing film was in the crystal state 
was 10% and that when the writing film was in the amorphous state was 30%. 
The return light dose Ic from the phase transition optical disk 5, in a 
crystal state of the writing film, during reading was 0.6 V, and the 
return light dose IA from the phase transition optical disk 5 in the 
amorphous state was 1.8 V. 
In the same way as above, verifying was carried out by varying the 
reference voltage CLV and reference voltage CTV, and checking was made as 
to the optimum values of the reference voltages CLV and CTV. As a result, 
it was found that, with respect to the reference voltage CLV, the writing 
error generating times detected were in accord with those in the prior art 
example when the condition (0.7.times.Ic.times.Pw)/Pr&lt;CLV&lt;(Ic.times.Pw)/Pr 
was satisfied. Also, with respect to the reference voltage CTV, the 
writing error generating times detected were in accord with those in the 
prior art example when the condition (Ic.times.PE)/Pr&lt;CTV&lt;(IA.times.PE)/Pr 
was satisfied. 
As explained above, for carrying out verification, it is necessary to 
confirm whether the laser light of a predetermined intensity has in fact 
irradiated the writing film, or the writing film has in fact been melted. 
For confirming whether or not the light of the predetermined intensity has 
irradiated, the intensity monitor output IL of the return light from the 
optical disk at the rise of the writing signal may be checked. Also, for 
confirming whether or not the writing film has melted, the light dose 
monitor output IT of the return light from the optical disk at the fall of 
the writing signal may be checked. In the optical disk satisfying an Rc&gt;Ra 
condition, the relationship among the reflectivities to be satisfied is 
Rc&gt;Rm&gt;Ra, and in the optical disk satisfying an Rc&lt;Ra condition, the 
relationship among the reflectivities to be satisfied is Rc&lt;Rm&lt;Ra, wherein 
Rc is reflectivity of the writing film when it is in a crystal state, Ra 
is reflectivity when same is in an amorphous state, and Rm is reflectivity 
when same is in a melted state. The intensity of the light dose monitor 
output IT may be expressed by IT=PE.times.Pm, wherein erase power is 
represented by PE. Thus, by checking whether relationships Rc&lt;(IT/PE)&lt;Ra 
or Ra&lt;(IT/PE)&lt;Rc have been satisfied, it is possible to confirm whether 
the writing film has melted. 
In each of the examples explained above, since verification can be carried 
out while writing is effected using a single beam spot, it is possible to 
reduce the time required for writing and also to simplify adjustment of 
the optical system which is otherwise complicated, thereby permitting 
manufacturing cost to be reduced. 
Now, a preferred embodiment of the second aspect of the invention is 
explained with reference to the drawings. 
FIG. 7 shows in a sectional view structure of an optical data writing 
medium according to the invention. As shown therein, on a transparent 
substrate 121, there are deposited a first protective film 122, writing 
film 123, second protective film 124 and reflection film 125. On the 
reflection film 125, ultraviolet-setting resin 126 is coated for 
protective purposes. 
The substrate 121 employs glass or plastic in a disk shape. The first 
protective film 122 and the second protective film 124 employ such 
dielectric materials as SiO.sub.2, Si.sub.3 N.sub.4, AlN, TiO.sub.2, ZnS, 
and Zns--SiO.sub.2. The writing film 123 employs GeSbTe, ZnSbTe, Inse, 
InTe, AsTeGe, TeOx--GeSn, TeSeSn, SbSeBi, BiSeGe, etc. of chalcogenide 
system. In the structure where transmittive high refraction materials are 
used as the reflection film 125, Si and Ge are used in particular. Also, 
when metal film is used as the reflection film 125, Al, Au or alloys of 
AlTi or alloys of AuPd, etc. can be used. 
A feature of this second aspect of the invention resides in that, of three 
reflectivities of the data writing film, respectively, in a crystal state, 
amorphous state, and melted state, reflectivity in the crystal state and 
reflectivity in the melted state are different from each other, and 
reflectivity in the amorphous state and reflectivity in the melted state 
are different from each other. 
FIGS. 8A, 8B and 8C are for use in explaining a writing/reading method, 
particularly a verifying operation during writing, by using an optical 
data writing medium according to the invention. By irradiation of laser 
light 130 during writing, the writing film 133 is heated to above the 
melting point. The numeral 131 in FIG. 8C indicates a region where the 
writing film has been heated to above the melting point. The monitored 
signals of the reflection light doses then obtained are divided into two 
kinds in their states, as shown respectively in FIG. 8A and FIG. 8B. That 
is, the state in FIG. 8A is a state wherein reflectivity of the melt 
region 131 of the writing film is higher than that of region 132 that 
immediately precedes the melt region 131, and the state shown in FIG. 8B 
is a state wherein reflectivity of the melt region 131 is lower than that 
of region 132 that immediately precedes the melt region 131. 
For verification operation during writing, that is, "simultaneous writing 
verification", it is important that the melt region 131 described above be 
accurately and correctly monitored. That is, during monitoring of the 
reflected light doses, it is necessary for reflectivity from the melt 
region and reflectivity from the region 132 immediately preceding the melt 
region 131 to be distinguishable with sufficient S/N ratio. For this 
reason, it is desired that the monitored signals be in the waveforms shown 
under FIGS. 8A and 8B. 
Here, the reasons for the monitor signals resulting in the waveforms shown 
in FIGS. 8A and 8B are that, with writing power irradiation, light 
reflection dose becomes higher during the time period corresponding to the 
writing pulse irradiation, there is a time lag between the writing power 
irradiation start time and the melt start time, and the laser light 130 
has a constant beam diameter. 
FIG. 9 is a graph showing the reflectivities obtained for an amorphous 
state, crystal state and melt state by one structure of the optical data 
writing medium according to the invention. The structure includes a 
transparent substrate 121, first protective film 122 formed on the 
transparent substrate 121, phase transition type data writing film 123 
formed on the first protective film 122, second protective film 124 formed 
on the writing film 123, and reflection film 125 of high refraction 
dielectric formed on the second protective film 124. It also includes a 
layer of ultraviolet-setting resin 126. As the high refraction dielectric 
reflection film 125, Si is used in this structure. Wavelength is 690 nm. 
Here, the transparent substrate employs polycarbonate, and the first 
protective film employs ZnS--SiO.sub.2. In the layered structure, the 
thickness of the phase transition type data writing film of GeSbTe is 10 
nm, that of the second protective film of ZnS--SiO.sub.2 is 18 nm, and 
that of the reflective film of Si is 60 nm. Further, a ZnS--SiO.sub.2 
layer of 120 nm thickness as an interference layer and an 
ultraviolet-setting resin layer of 10 .mu.m are formed on the resulting 
structure. 
Where the thicknesses of the first protective film are between 70 and 130 
nm and between 230 and 280 nm, the reflectivity of the crystal state and 
that of the melt state become essentially the same so that it is difficult 
to distinguish one from the other. However, in the remaining regions of 
the first protective film, the reflectivities, respectively, of the three 
states, namely, the crystal, amorphous and melt states, are distinctly 
different from one another, thereby making it possible to distinguish the 
melt state from the other states. 
FIG. 10 is a graph showing the reflectivities obtained for an amorphous 
state, crystal state and melt state in another structure of the optical 
data writing medium according to the invention. The structure includes a 
transparent substrate 121, first protective film 122 formed on the 
transparent substrate 121, phase transition type data writing film 123 
formed on the first protective film 122, second protective film 124 formed 
on the writing film, and reflection film 125 of metal formed on the second 
protective film 124. It also includes a layer of ultraviolet-setting resin 
126. As the metal reflection film 125, Al is used in this structure. 
Wavelength is 690 nm. 
Here, the transparent substrate employs polycarbonate, and the first 
protective film employs ZnS--SiO.sub.2. In the layered structure, the 
thickness of the phase transition type data 8 writing film of GeSbTe is 12 
nm, that of the second protective film of ZnS--SiO.sub.2 is 20 nm, and 
that of the reflective film of Al is 60 nm. Further, an 
ultraviolet-setting resin layer of 10 .mu.m is formed on the resulting 
structure. 
As shown in FIG. 10, within the extent of the thicknesses of the first 
protective film, up to 300 nm, the reflectivities in the three states, 
that is, crystal, amorphous and melt states, are distinctly different from 
one another in the neighborhood of 130 nm, 210 nm, and 300 nm of the 
thicknesses of the first protective film, and this enables confirmation of 
the melt state. 
FIG. 11 is a graph showing the reflectivities obtained for an amorphous 
state, crystal state and melt state with respect to still another 
structure of the optical data writing medium according to the invention. 
The structure includes a transparent substrate 121, first protective film 
122 formed on the transparent substrate 121, phase transition type data 
writing film 123 formed on the first protective film 122, second 
protective film 124 formed on the writing film 123, and reflection film 
125 of metal formed on the second protective film 124. It also includes a 
layer of ultraviolet-setting resin 126. As the metal reflection film 125, 
a thin film of Au is used in this structure. Wavelength is 690 nm. 
Here, the transparent substrate employs polycarbonate, and the first 
protective film employs ZnS--SiO.sub.2. In the layered structure, the 
thickness of the phase transition type data writing film of GeSbTe is 40 
nm, that of the second protective film of ZnS--SiO.sub.2 is 140 nm, and 
that of the reflective film of Au is 10 nm. Further, an 
ultraviolet-setting resin layer of 10 .mu.m is formed on the resulting 
structure. 
As shown in FIG. 11, the reflectivities in the amorphous and melt states 
are approximately the same and not distinguishable in the neighborhood of 
100 nm and 260 nm of the thicknesses of the first protective film, but the 
reflectivities in the three states are significantly different from one 
another where the first protective film has thicknesses other than above 
thicknesses, thus enabling confirmation of the melt state. 
In all of the above structures, it is the setting of the thicknesses of the 
medium that has made it possible to realize the desired three reflectivity 
characteristics, namely, the reflectivity characteristics under the 
crystal state, those under the amorphous state and those under the melt 
state of the data writing film. 
For confirming the effects of this aspect of the invention, various tests 
have been conducted as explained below. 
EXAMPLE 4 
The optical data writing medium for this test is prepared as follows: The 
transparent substrate employs a polycarbonate substrate having preformed 
grooves and having a diameter of 130 mm and thickness of 1.2 mm. By 
sputtering, there are formed a ZnS--SiO.sub.2 film of 150 nm thickness as 
a first protective film, a Ge.sub.2 Sb.sub.2 Te.sub.5 film of 10 nm 
thickness as a writing film, a ZnS--SiO.sub.2 film of 18 nm thickness as a 
second protective film, and an Si reflection film of 60 nm thickness as a 
reflection film. Also, a ZnS--SiO.sub.2 film of 120 nm thickness is formed 
as an interference layer. Further, by spin coating, an ultraviolet-setting 
resin film of 10 .mu.m thickness is deposited. 
Then, by using an optical head mounted with a laser diode with a wavelength 
of 690 nm, overwrite is carried out and the reproduced waveforms during 
writing are monitored. The reproduced waveforms during writing have shown 
changes in reflectivity in the melt portion, thus enabling confirmation of 
the writing state as being in order. 
COMISON EXAMPLE 1 
The optical data writing medium for this comparison test is prepared as 
follows: The transparent substrate employs a polycarbonate substrate 
having preformed-grooves and having a diameter of 130 mm and thickness of 
1.2 mm. By sputtering, there are formed a ZnS--SiO.sub.2 film of 100 nm 
thickness as a first protective film, a Ge.sub.2 Sb.sub.2 Te.sub.5 film of 
10 nm thickness as a writing film, a ZnS--SiO.sub.2 film of 18 nm 
thickness as a second protective film, and an Si film of 60 nm thickness 
as a reflection film. Also, a ZnS--SiO.sub.2 film of 120 nm thickness is 
formed as an interference layer. Further, by spin coating, an 
ultraviolet-setting resin film of 10 .mu.m thickness is deposited. 
Then, by using an optical head mounted with a laser diode having a 
wavelength of 690 nm, overwrite is carried out and reproduced waveforms 
during writing are monitored. From these reproduced waveforms, it has not 
been possible to observe a distinction between the melt portion and the 
crystal portion, that is, it has not been possible to confirm the 
correctness of the writing state. 
EXAMPLE 5 
The optical data writing medium for this test is prepared as follows: The 
transparent substrate employs a polycarbonate substrate having 
preformed-grooves and having a diameter of 130 mm and thickness of 1.2 mm. 
By sputtering, there are formed a ZnS--SiO.sub.2 film of 130 nm thickness 
as a first protective film, a Ge.sub.2 Sb.sub.2 Te.sub.5 film of 12 nm 
thickness as a writing film, a ZnS--SiO.sub.2 film of 20 nm thickness as a 
second protective film, and an Al film of 60 nm thickness as a reflection 
film. Also, by spin coating, an ultraviolet-setting resin film of 10 .mu.m 
thickness is deposited. 
Then, by using an optical head mounted with a laser diode having a 
wavelength of 690 nm, overwrite is carried out and reproduced waveforms 
during writing are monitored. The reproduced waveforms during writing have 
shown an increase in reflectivity in the melt portion as compared with 
that in the crystal portion, thus enabling confirmation of the writing 
state as being in order. 
COMISON EXAMPLE 2 
The optical data writing medium for this comparison test is prepared as 
follows: The transparent substrate employs a polycarbonate substrate 
having preformed-grooves and having a diameter of 130 mm and thickness of 
1.2 mm. By sputtering, there are formed a ZnS--SiO.sub.2 film of 180 nm 
thickness as a first protective film, a Ge.sub.2 Sb.sub.2 Te.sub.5 film of 
12 nm thickness as a writing film, a ZnS--SiO.sub.2 film of 20 nm 
thickness as a second protective film, and an Al film of 60 nm thickness 
as a reflection film. Further, by spin coating, an ultraviolet-setting 
resin film of 10 .mu.m thickness is deposited. 
Then, by using an optical head mounted with a laser diode having a 
wavelength of 690 nm, overwrite is carried out and reproduced waveforms 
during writing are monitored. From these reproduced waveforms, it has not 
been possible to observe a distinction between the melt portion and 
crystal portion, that is, it has not been possible to confirm the 
correctness of the writing state. 
EXAMPLE 6 
The optical data writing medium for this test is prepared as follows: The 
transparent substrate employs a polycarbonate substrate having 
preformed-grooves and having a diameter of 130 mm and thickness of 1.2 mm. 
By sputtering, there are formed a ZnS--SiO.sub.2 film of 180 nm thickness 
as a first protective film, a Ge.sub.2 Sb.sub.2 Te.sub.5 film of 40 nm 
thickness as a writing film, a ZnS--SiO.sub.2 film of 140 nm thickness as 
a second protective film, and an Au film of 10 nm thickness as a 
reflection film. Further, by spin coating, an ultraviolet-setting resin 
film of 10 .mu.m thickness is deposited. 
Then, by using an optical head mounted with a laser diode having a 
wavelength of 690 nm, overwrite is carried out and reproduced waveforms 
during writing are monitored. The reproduced waveforms during writing have 
shown a decrease in reflectivity in the melt portion as compared with that 
in the amorphous portion, thus enabling confirmation of the writing state 
as being in order. 
COMISON EXAMPLE 3 
The optical data writing medium for this comparison test is prepared as 
follows: The transparent substrate employs a polycarbonate substrate 
having preformed-grooves and having a diameter of 130 mm and thickness of 
1.2 mm. By sputtering, there are formed a ZnS--SiO.sub.2 film of 110 nm 
thickness as a first protective film, a Ge.sub.2 Sb.sub.2 Te.sub.5 film of 
40 nm thickness as a writing film, a ZnS--SiO.sub.2 film of 140 nm 
thickness as a second protective film, and an Au film of 10 nm thickness 
as a reflection film. Further, by spin coating, an ultraviolet-setting 
resin film of 10 .mu.m thickness is deposited. 
Then, by using an optical head mounted with a laser diode having a 
wavelength of 690 nm, overwrite is carried out and reproduced waveforms 
during writing are monitored. From these reproduced waveforms, it has not 
been possible to observe a distinction between the melt portion and 
amorphous portion, that is, it has not been possible to confirm the 
correctness of the writing state. 
EXAMPLE 7 
The optical data writing medium for this test is prepared by using Ge for 
the reflection film instead of Si used in Example 4 with the remaining 
being the same as in Example 4. 
Then, by using an optical head mounted with a laser diode having a 
wavelength of 690 nm, overwrite is carried out and reproduced waveforms 
during writing are monitored. The reproduced waveforms during writing have 
shown changes in reflectivity in the melt portion, thus enabling 
confirmation of the writing state as being in order. 
EXAMPLE 8 
The optical data writing medium for this test is prepared by using Al--Ti 
for the reflection film instead of Al used in Example 5 with the remaining 
being the same as in Example 5. 
By using an optical head mounted with a laser diode having a wavelength of 
690 nm, overwrite is carried out and reproduced waveforms during writing 
are monitored. The reproduced waveforms during writing have shown an 
increase in reflectivity in the melt portion as compared with that in the 
crystal portion, thus enabling confirmation of the writing state as being 
in order. 
EXAMPLE 9 
In order to confirm the effect of the writing/reading method according to 
the invention, by using the optical data writing medium described with 
reference to Example 4, the verifying operation during writing has been 
evaluated for confirming the effect of the writing/reading method 
according to the invention. 
For judgment of the verifying operation, a judgment circuit illustrated in 
FIG. 12 has been used. Here, comparator 144 receives reading signal 141 
during writing and comparator level 142, and it outputs comparator output 
signal 143. In the arrangement shown in FIG. 12, the comparator input 
level with respect to the reading signal during the writing has been set 
as shown by 142 in FIG. 13. 
FIG. 13 shows the waveforms obtained when writing is made under the crystal 
state, waveform 140 showing the writing signal, waveform 141 showing the 
reading signal during writing, and waveform 142 showing verifying 
comparator level. In this example, since reflectivity in the melt state is 
lower than that in the crystal state, the comparator output pulses are as 
shown by waveform 143. Thus, the crystal state and melt state can be 
distinguished from each other whereby a desired verifying output can be 
obtained. 
FIG. 14 shows the waveforms obtained when writing is made under the 
amorphous state, waveform 140 showing the writing signal, waveform 141 
showing the reading signal during writing, and waveform 142 showing the 
verifying comparator level. In this example, since reflectivity in the 
melt state is lower than that in the crystal state, the comparator output 
pulses are as shown by waveform 143. Thus, the crystal state and melt 
state can be distinguished from each other whereby a desired verifying 
output can be obtained. 
As explained above, the invention provides a structure of the medium which 
is suited for simultaneous writing/verifying operations of the phase 
transition type disk, and it also provides a writing/reading method which 
is featured in simultaneous writing/verifying operations. According to the 
invention, since it is possible to realize a structure of the medium in 
which reflectivities in the melt, crystal and amorphous states are 
distinctly different from one another, and simultaneous writing/verifying 
procedures which are suited for such medium, a great advantage is 
engendered by the invention in that data rewriting operations can be 
effected at high speed. Another great advantage is that an overwrite 
operation can be completed within one disk rotation. 
While the invention has been described with reference to its preferred 
embodiments, it is to be understood that the words which have been used 
are words of description rather than limitation and that changes within 
the purview of the appended claims may be made without departing from the 
true scope of the invention as defined by the claims.